active_support_core_extensions.md

Active Support Core Extensions
==============================

Active Support is the Ruby on Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff.

It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Ruby on Rails itself.

After reading this guide, you will know:

* What Core Extensions are.
* How to load all extensions.
* How to cherry-pick just the extensions you want.
* What extensions ActiveSupport provides.

--------------------------------------------------------------------------------

How to Load Core Extensions
---------------------------

### Stand-Alone Active Support

In order to have a near-zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you can load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

```ruby
require 'active_support'
```

objects do not even respond to `blank?`. Let's see how to load its definition.

#### Cherry-picking a Definition

The most lightweight way to get `blank?` is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where such a method is defined. In the case of `blank?` the note reads:

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

That means that this single call is enough:

```ruby
require 'active_support/core_ext/object/blank'
```

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

#### Loading Grouped Core Extensions

The next level is to simply load all extensions to `Object`. As a rule of thumb, extensions to `SomeClass` are available in one shot by loading `active_support/core_ext/some_class`.

Thus, to load all extensions to `Object` (including `blank?`):

```ruby
require 'active_support/core_ext/object'
```

#### Loading All Core Extensions

You may prefer just to load all core extensions, there is a file for that:

```ruby
require 'active_support/core_ext'
```

#### Loading All Active Support

And finally, if you want to have all Active Support available just issue:

```ruby
require 'active_support/all'
```

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via `autoload`, so it is only loaded if used.

### Active Support Within a Ruby on Rails Application

A Ruby on Rails application loads all Active Support unless `config.active_support.bare` is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

Extensions to All Objects
-------------------------

### `blank?` and `present?`

The following values are considered to be blank in a Rails application:

* `nil` and `false`,

* strings composed only of whitespace (see note below),

* empty arrays and hashes, and

* any other object that responds to `empty?` and is empty.

INFO: The predicate for strings uses the Unicode-aware character class `[:space:]`, so for example U+2029 (paragraph separator) is considered to be whitespace.

WARNING: Note that numbers are not mentioned. In particular, 0 and 0.0 are **not** blank.

For example, this method from `ActionDispatch::Session::AbstractStore` uses `blank?` for checking whether a session key is present:

```ruby
def ensure_session_key!
  if @key.blank?
    raise ArgumentError, 'A key is required...'
  end
end
```

The method `present?` is equivalent to `!blank?`. This example is taken from `ActionDispatch::Http::Cache::Response`:

```ruby
def set_conditional_cache_control!
  return if self["Cache-Control"].present?
  ...
end
```

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

### `presence`

The `presence` method returns its receiver if `present?`, and `nil` otherwise. It is useful for idioms like this:

```ruby
host = config[:host].presence || 'localhost'
```

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

### `duplicable?`

A few fundamental objects in Ruby are singletons. For example, in the whole life of a program the integer 1 refers always to the same instance:

```ruby
1.object_id                 # => 3
Math.cos(0).to_i.object_id  # => 3
```

Hence, there's no way these objects can be duplicated through `dup` or `clone`:

```ruby
true.dup  # => TypeError: can't dup TrueClass
```

Some numbers which are not singletons are not duplicable either:

```ruby
0.0.clone        # => allocator undefined for Float
(2**1024).clone  # => allocator undefined for Bignum
```

Active Support provides `duplicable?` to programmatically query an object about this property:

```ruby
"foo".duplicable? # => true
"".duplicable?     # => true
0.0.duplicable?   # => false
false.duplicable?  # => false
```

By definition all objects are `duplicable?` except `nil`, `false`, `true`, symbols, numbers, class, and module objects.

WARNING: Any class can disallow duplication by removing `dup` and `clone` or raising exceptions from them. Thus only `rescue` can tell whether a given arbitrary object is duplicable. `duplicable?` depends on the hard-coded list above, but it is much faster than `rescue`. Use it only if you know the hard-coded list is enough in your use case.

NOTE: Defined in `active_support/core_ext/object/duplicable.rb`.

### `deep_dup`

The `deep_dup` method returns deep copy of a given object. Normally, when you `dup` an object that contains other objects, ruby does not `dup` them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:

```ruby
array     = ['string']
duplicate = array.dup

duplicate.push 'another-string'

# the object was duplicated, so the element was added only to the duplicate
array     #=> ['string']
duplicate #=> ['string', 'another-string']

duplicate.first.gsub!('string', 'foo')

# first element was not duplicated, it will be changed in both arrays
array     #=> ['foo']
duplicate #=> ['foo', 'another-string']
```

As you can see, after duplicating the `Array` instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since `dup` does not make deep copy, the string inside the array is still the same object.

If you need a deep copy of an object, you should use `deep_dup`. Here is an example:

```ruby
array     = ['string']
duplicate = array.deep_dup

duplicate.first.gsub!('string', 'foo')

array     #=> ['string']
duplicate #=> ['foo']
```

If the object is not duplicable, `deep_dup` will just return it:

```ruby
number = 1
duplicate = number.deep_dup
number.object_id == duplicate.object_id   # => true
```

NOTE: Defined in `active_support/core_ext/object/deep_dup.rb`.

### `try`

When you want to call a method on an object only if it is not `nil`, the simplest way to achieve it is with conditional statements, adding unnecessary clutter. The alternative is to use `try`. `try` is like `Object#send` except that it returns `nil` if sent to `nil`.

Here is an example:

```ruby
# without try
unless @number.nil?
  @number.next
end

# with try
@number.try(:next)
```

Another example is this code from `ActiveRecord::ConnectionAdapters::AbstractAdapter` where `@logger` could be `nil`. You can see that the code uses `try` and avoids an unnecessary check.

```ruby
def log_info(sql, name, ms)
  if @logger.try(:debug?)
    name = '%s (%.1fms)' % [name || 'SQL', ms]
    @logger.debug(format_log_entry(name, sql.squeeze(' ')))
  end
end
```

`try` can also be called without arguments but a block, which will only be executed if the object is not nil:

```ruby
@person.try { |p| "#{p.first_name} #{p.last_name}" }
```

NOTE: Defined in `active_support/core_ext/object/try.rb`.

### `class_eval(*args, &block)`

You can evaluate code in the context of any object's singleton class using `class_eval`:

```ruby
class Proc
  def bind(object)
    block, time = self, Time.current
    object.class_eval do
      method_name = "__bind_#{time.to_i}_#{time.usec}"
      define_method(method_name, &block)
      method = instance_method(method_name)
      remove_method(method_name)
      method
    end.bind(object)
  end
end
```

NOTE: Defined in `active_support/core_ext/kernel/singleton_class.rb`.

### `acts_like?(duck)`

The method `acts_like?` provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as `String` defines

```ruby
def acts_like_string?
end
```

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:

```ruby
some_klass.acts_like?(:string)
```

Rails has classes that act like `Date` or `Time` and follow this contract.

NOTE: Defined in `active_support/core_ext/object/acts_like.rb`.

### `to_param`

All objects in Rails respond to the method `to_param`, which is meant to return something that represents them as values in a query string, or as URL fragments.

By default `to_param` just calls `to_s`:

```ruby
7.to_param # => "7"
```

The return value of `to_param` should **not** be escaped:

```ruby
"Tom & Jerry".to_param # => "Tom & Jerry"
```

Several classes in Rails overwrite this method.

For example `nil`, `true`, and `false` return themselves. `Array#to_param` calls `to_param` on the elements and joins the result with "/":

```ruby
[0, true, String].to_param # => "0/true/String"
```

Notably, the Rails routing system calls `to_param` on models to get a value for the `:id` placeholder. `ActiveRecord::Base#to_param` returns the `id` of a model, but you can redefine that method in your models. For example, given

```ruby
class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end
```

we get:

```ruby
user_path(@user) # => "/users/357-john-smith"
```

WARNING. Controllers need to be aware of any redefinition of `to_param` because when a request like that comes in "357-john-smith" is the value of `params[:id]`.

NOTE: Defined in `active_support/core_ext/object/to_param.rb`.

### `to_query`

Except for hashes, given an unescaped `key` this method constructs the part of a query string that would map such key to what `to_param` returns. For example, given

```ruby
class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end
```

we get:

```ruby
current_user.to_query('user') # => user=357-john-smith
```

This method escapes whatever is needed, both for the key and the value:

```ruby
account.to_query('company[name]')
# => "company%5Bname%5D=Johnson+%26+Johnson"
```

so its output is ready to be used in a query string.

Arrays return the result of applying `to_query` to each element with `_key_[]` as key, and join the result with "&":

```ruby
[3.4, -45.6].to_query('sample')
# => "sample%5B%5D=3.4&sample%5B%5D=-45.6"
```

Hashes also respond to `to_query` but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling `to_query(key)` on its values. Then it joins the result with "&":

```ruby
{c: 3, b: 2, a: 1}.to_query # => "a=1&b=2&c=3"
```

The method `Hash#to_query` accepts an optional namespace for the keys:

```ruby
{id: 89, name: "John Smith"}.to_query('user')
# => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"
```

NOTE: Defined in `active_support/core_ext/object/to_query.rb`.

### `with_options`

The method `with_options` provides a way to factor out common options in a series of method calls.

Given a default options hash, `with_options` yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

```ruby
class Account < ActiveRecord::Base
  has_many :customers, dependent: :destroy
  has_many :products,  dependent: :destroy
  has_many :invoices,  dependent: :destroy
  has_many :expenses,  dependent: :destroy
end
```

this way:

```ruby
class Account < ActiveRecord::Base
  with_options dependent: :destroy do |assoc|
    assoc.has_many :customers
    assoc.has_many :products
    assoc.has_many :invoices
    assoc.has_many :expenses
  end
end
```

That idiom may convey _grouping_ to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

```ruby
I18n.with_options locale: user.locale, scope: "newsletter" do |i18n|
  subject i18n.t :subject
  body    i18n.t :body, user_name: user.name
end
```

TIP: Since `with_options` forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

NOTE: Defined in `active_support/core_ext/object/with_options.rb`.

### Instance Variables

Active Support provides several methods to ease access to instance variables.

#### `instance_values`

The method `instance_values` returns a hash that maps instance variable names without "@" to their
corresponding values. Keys are strings:

```ruby
class C
  def initialize(x, y)
    @x, @y = x, y
  end
end

C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}
```

NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.

### Silencing Warnings, Streams, and Exceptions

The methods `silence_warnings` and `enable_warnings` change the value of `$VERBOSE` accordingly for the duration of their block, and reset it afterwards:

```ruby
silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }
```

You can silence any stream while a block runs with `silence_stream`:

```ruby
silence_stream(STDOUT) do
  # STDOUT is silent here
end
```

The `quietly` method addresses the common use case where you want to silence STDOUT and STDERR, even in subprocesses:

```ruby
quietly { system 'bundle install' }
```

For example, the railties test suite uses that one in a few places to prevent command messages from being echoed intermixed with the progress status.

Silencing exceptions is also possible with `suppress`. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is `kind_of?` any of the arguments, `suppress` captures it and returns silently. Otherwise the exception is reraised:

```ruby
# If the user is locked the increment is lost, no big deal.
suppress(ActiveRecord::StaleObjectError) do
  current_user.increment! :visits
end
```

NOTE: Defined in `active_support/core_ext/kernel/reporting.rb`.

### `in?`

The predicate `in?` tests if an object is included in another object or a list of objects. An `ArgumentError` exception will be raised if a single argument is passed and it does not respond to `include?`.

Examples of `in?`:

```ruby
1.in?(1,2)          # => true
1.in?([1,2])        # => true
"lo".in?("hello")   # => true
25.in?(30..50)      # => false
1.in?(1)            # => ArgumentError
```

NOTE: Defined in `active_support/core_ext/object/inclusion.rb`.

Extensions to `Module`
----------------------

### `alias_method_chain`

Using plain Ruby you can wrap methods with other methods, that's called _alias chaining_.

For example, let's say you'd like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap `ActionController::TestCase#process` this way in `test/test_helper.rb`:

```ruby
ActionController::TestCase.class_eval do
  # save a reference to the original process method
  alias_method :original_process, :process

  # now redefine process and delegate to original_process
  def process(action, params=nil, session=nil, flash=nil, http_method='GET')
    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
    original_process(action, params, session, flash, http_method)
  end
end
```

That's the method `get`, `post`, etc., delegate the work to.

That technique has a risk, it could be the case that `:original_process` was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:

```ruby
ActionController::TestCase.class_eval do
  def process_with_stringified_params(...)
    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
    process_without_stringified_params(action, params, session, flash, http_method)
  end
  alias_method :process_without_stringified_params, :process
  alias_method :process, :process_with_stringified_params
end
```

The method `alias_method_chain` provides a shortcut for that pattern:

```ruby
ActionController::TestCase.class_eval do
  def process_with_stringified_params(...)
    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
    process_without_stringified_params(action, params, session, flash, http_method)
  end
  alias_method_chain :process, :stringified_params
end
```

Rails uses `alias_method_chain` all over the code base. For example validations are added to `ActiveRecord::Base#save` by wrapping the method that way in a separate module specialized in validations.

NOTE: Defined in `active_support/core_ext/module/aliasing.rb`.

### Attributes

#### `alias_attribute`

Model attributes have a reader, a writer, and a predicate. You can alias a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (my mnemonic is they go in the same order as if you did an assignment):

```ruby
class User < ActiveRecord::Base
  # let me refer to the email column as "login",
  # possibly meaningful for authentication code
  alias_attribute :login, :email
end
```

NOTE: Defined in `active_support/core_ext/module/aliasing.rb`.

#### Internal Attributes

When you are defining an attribute in a class that is meant to be subclassed, name collisions are a risk. That's remarkably important for libraries.

Active Support defines the macros `attr_internal_reader`, `attr_internal_writer`, and `attr_internal_accessor`. They behave like their Ruby built-in `attr_*` counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro `attr_internal` is a synonym for `attr_internal_accessor`:

```ruby
# library
class ThirdPartyLibrary::Crawler
  attr_internal :log_level
end

# client code
class MyCrawler < ThirdPartyLibrary::Crawler
  attr_accessor :log_level
end
```

In the previous example it could be the case that `:log_level` does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own `:log_level`. Thanks to `attr_internal` there's no collision.

By default the internal instance variable is named with a leading underscore, `@_log_level` in the example above. That's configurable via `Module.attr_internal_naming_format` though, you can pass any `sprintf`-like format string with a leading `@` and a `%s` somewhere, which is where the name will be placed. The default is `"@_%s"`.

Rails uses internal attributes in a few spots, for examples for views:

```ruby
module ActionView
  class Base
    attr_internal :captures
    attr_internal :request, :layout
    attr_internal :controller, :template
  end
end
```

NOTE: Defined in `active_support/core_ext/module/attr_internal.rb`.

#### Module Attributes

The macros `mattr_reader`, `mattr_writer`, and `mattr_accessor` are analogous to the `cattr_*` macros defined for class. Check [Class Attributes](#class-attributes).

For example, the dependencies mechanism uses them:

```ruby
module ActiveSupport
  module Dependencies
    mattr_accessor :warnings_on_first_load
    mattr_accessor :history
    mattr_accessor :loaded
    mattr_accessor :mechanism
    mattr_accessor :load_paths
    mattr_accessor :load_once_paths
    mattr_accessor :autoloaded_constants
    mattr_accessor :explicitly_unloadable_constants
    mattr_accessor :logger
    mattr_accessor :log_activity
    mattr_accessor :constant_watch_stack
    mattr_accessor :constant_watch_stack_mutex
  end
end
```

NOTE: Defined in `active_support/core_ext/module/attribute_accessors.rb`.

### Parents

#### `parent`

The `parent` method on a nested named module returns the module that contains its corresponding constant:

```ruby
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent # => X::Y
M.parent       # => X::Y
```

If the module is anonymous or belongs to the top-level, `parent` returns `Object`.

WARNING: Note that in that case `parent_name` returns `nil`.

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### `parent_name`

The `parent_name` method on a nested named module returns the fully-qualified name of the module that contains its corresponding constant:

```ruby
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent_name # => "X::Y"
M.parent_name       # => "X::Y"
```

For top-level or anonymous modules `parent_name` returns `nil`.

WARNING: Note that in that case `parent` returns `Object`.

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### `parents`

The method `parents` calls `parent` on the receiver and upwards until `Object` is reached. The chain is returned in an array, from bottom to top:

```ruby
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parents # => [X::Y, X, Object]
M.parents       # => [X::Y, X, Object]
```

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

### Constants

The method `local_constants` returns the names of the constants that have been
defined in the receiver module:

```ruby
module X
  X1 = 1
  X2 = 2
  module Y
    Y1 = :y1
    X1 = :overrides_X1_above
  end
end

X.local_constants    # => [:X1, :X2, :Y]
X::Y.local_constants # => [:Y1, :X1]
```

The names are returned as symbols. (The deprecated method `local_constant_names` returns strings.)

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### Qualified Constant Names

The standard methods `const_defined?`, `const_get` , and `const_set` accept
bare constant names. Active Support extends this API to be able to pass
relative qualified constant names.

The new methods are `qualified_const_defined?`, `qualified_const_get`, and
`qualified_const_set`. Their arguments are assumed to be qualified constant
names relative to their receiver:

```ruby
Object.qualified_const_defined?("Math::PI")       # => true
Object.qualified_const_get("Math::PI")            # => 3.141592653589793
Object.qualified_const_set("Math::Phi", 1.618034) # => 1.618034
```

Arguments may be bare constant names:

```ruby
Math.qualified_const_get("E") # => 2.718281828459045
```

These methods are analogous to their builtin counterparts. In particular,
`qualified_constant_defined?` accepts an optional second argument to be
able to say whether you want the predicate to look in the ancestors.
This flag is taken into account for each constant in the expression while
walking down the path.

For example, given

```ruby
module M
  X = 1
end

module N
  class C
    include M
  end
end
```

`qualified_const_defined?` behaves this way:

```ruby
N.qualified_const_defined?("C::X", false) # => false
N.qualified_const_defined?("C::X", true)  # => true
N.qualified_const_defined?("C::X")        # => true
```

As the last example implies, the second argument defaults to true,
as in `const_defined?`.

For coherence with the builtin methods only relative paths are accepted.
Absolute qualified constant names like `::Math::PI` raise `NameError`.

NOTE: Defined in `active_support/core_ext/module/qualified_const.rb`.

### Reachable

A named module is reachable if it is stored in its corresponding constant. It means you can reach the module object via the constant.

That is what ordinarily happens, if a module is called "M", the `M` constant exists and holds it:

```ruby
module M
end

M.reachable? # => true
```

But since constants and modules are indeed kind of decoupled, module objects can become unreachable:

```ruby
module M
end

orphan = Object.send(:remove_const, :M)

# The module object is orphan now but it still has a name.
orphan.name # => "M"

# You cannot reach it via the constant M because it does not even exist.
orphan.reachable? # => false

# Let's define a module called "M" again.
module M
end

# The constant M exists now again, and it stores a module
# object called "M", but it is a new instance.
orphan.reachable? # => false
```

NOTE: Defined in `active_support/core_ext/module/reachable.rb`.

### Anonymous

A module may or may not have a name:

```ruby
module M
end
M.name # => "M"

N = Module.new
N.name # => "N"

Module.new.name # => nil
```

You can check whether a module has a name with the predicate `anonymous?`:

```ruby
module M
end
M.anonymous? # => false

Module.new.anonymous? # => true
```

Note that being unreachable does not imply being anonymous:

```ruby
module M
end

m = Object.send(:remove_const, :M)

m.reachable? # => false
m.anonymous? # => false
```

though an anonymous module is unreachable by definition.

NOTE: Defined in `active_support/core_ext/module/anonymous.rb`.

### Method Delegation

The macro `delegate` offers an easy way to forward methods.

Let's imagine that users in some application have login information in the `User` model but name and other data in a separate `Profile` model:

```ruby
class User < ActiveRecord::Base
  has_one :profile
end
```

With that configuration you get a user's name via his profile, `user.profile.name`, but it could be handy to still be able to access such attribute directly:

```ruby
class User < ActiveRecord::Base
  has_one :profile

  def name
    profile.name
  end
end
```

That is what `delegate` does for you:

```ruby
class User < ActiveRecord::Base
  has_one :profile

  delegate :name, to: :profile
end
```

It is shorter, and the intention more obvious.

The method must be public in the target.

The `delegate` macro accepts several methods:

```ruby
delegate :name, :age, :address, :twitter, to: :profile
```

When interpolated into a string, the `:to` option should become an expression that evaluates to the object the method is delegated to. Typically a string or symbol. Such an expression is evaluated in the context of the receiver:

```ruby
# delegates to the Rails constant
delegate :logger, to: :Rails

# delegates to the receiver's class
delegate :table_name, to: :class
```

WARNING: If the `:prefix` option is `true` this is less generic, see below.

By default, if the delegation raises `NoMethodError` and the target is `nil` the exception is propagated. You can ask that `nil` is returned instead with the `:allow_nil` option:

```ruby
delegate :name, to: :profile, allow_nil: true
```

With `:allow_nil` the call `user.name` returns `nil` if the user has no profile.

The option `:prefix` adds a prefix to the name of the generated method. This may be handy for example to get a better name:

```ruby
delegate :street, to: :address, prefix: true
```

The previous example generates `address_street` rather than `street`.

WARNING: Since in this case the name of the generated method is composed of the target object and target method names, the `:to` option must be a method name.

A custom prefix may also be configured:

```ruby
delegate :size, to: :attachment, prefix: :avatar
```

In the previous example the macro generates `avatar_size` rather than `size`.

NOTE: Defined in `active_support/core_ext/module/delegation.rb`

### Redefining Methods

There are cases where you need to define a method with `define_method`, but don't know whether a method with that name already exists. If it does, a warning is issued if they are enabled. No big deal, but not clean either.

The method `redefine_method` prevents such a potential warning, removing the existing method before if needed. Rails uses it in a few places, for instance when it generates an association's API:

```ruby
redefine_method("#{reflection.name}=") do |new_value|
  association = association_instance_get(reflection.name)

  if association.nil? || association.target != new_value
    association = association_proxy_class.new(self, reflection)
  end

  association.replace(new_value)
  association_instance_set(reflection.name, new_value.nil? ? nil : association)
end
```

NOTE: Defined in `active_support/core_ext/module/remove_method.rb`

Extensions to `Class`
---------------------

### Class Attributes

#### `class_attribute`

The method `class_attribute` declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.

```ruby
class A
  class_attribute :x
end

class B < A; end

class C < B; end

A.x = :a
B.x # => :a
C.x # => :a

B.x = :b
A.x # => :a
C.x # => :b

C.x = :c
A.x # => :a
B.x # => :b
```

For example `ActionMailer::Base` defines:

```ruby
class_attribute :default_params
self.default_params = {
  mime_version: "1.0",
  charset: "UTF-8",
  content_type: "text/plain",
  parts_order: [ "text/plain", "text/enriched", "text/html" ]
}.freeze
```

They can be also accessed and overridden at the instance level.

```ruby
A.x = 1

a1 = A.new
a2 = A.new
a2.x = 2

a1.x # => 1, comes from A
a2.x # => 2, overridden in a2
```

The generation of the writer instance method can be prevented by setting the option `:instance_writer` to `false`.

```ruby
module ActiveRecord
  class Base
    class_attribute :table_name_prefix, instance_writer: false
    self.table_name_prefix = ""
  end
end
```

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

The generation of the reader instance method can be prevented by setting the option `:instance_reader` to `false`.

```ruby
class A
  class_attribute :x, instance_reader: false
end

A.new.x = 1 # NoMethodError
```

For convenience `class_attribute` also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called `x?`.

When `:instance_reader` is `false`, the instance predicate returns a `NoMethodError` just like the reader method.

NOTE: Defined in `active_support/core_ext/class/attribute.rb`

#### `cattr_reader`, `cattr_writer`, and `cattr_accessor`

The macros `cattr_reader`, `cattr_writer`, and `cattr_accessor` are analogous to their `attr_*` counterparts but for classes. They initialize a class variable to `nil` unless it already exists, and generate the corresponding class methods to access it:

```ruby
class MysqlAdapter < AbstractAdapter
  # Generates class methods to access @@emulate_booleans.
  cattr_accessor :emulate_booleans
  self.emulate_booleans = true
end
```

Instance methods are created as well for convenience, they are just proxies to the class attribute. So, instances can change the class attribute, but cannot override it as it happens with `class_attribute` (see above). For example given

```ruby
module ActionView
  class Base
    cattr_accessor :field_error_proc
    @@field_error_proc = Proc.new{ ... }
  end
end
```

we can access `field_error_proc` in views.

The generation of the reader instance method can be prevented by setting `:instance_reader` to `false` and the generation of the writer instance method can be prevented by setting `:instance_writer` to `false`. Generation of both methods can be prevented by setting `:instance_accessor` to `false`. In all cases, the value must be exactly `false` and not any false value.

```ruby
module A
  class B
    # No first_name instance reader is generated.
    cattr_accessor :first_name, instance_reader: false
    # No last_name= instance writer is generated.
    cattr_accessor :last_name, instance_writer: false
    # No surname instance reader or surname= writer is generated.
    cattr_accessor :surname, instance_accessor: false
  end
end
```

A model may find it useful to set `:instance_accessor` to `false` as a way to prevent mass-assignment from setting the attribute.

NOTE: Defined in `active_support/core_ext/class/attribute_accessors.rb`.

### Subclasses & Descendants

#### `subclasses`

The `subclasses` method returns the subclasses of the receiver:

```ruby
class C; end
C.subclasses # => []

class B < C; end
C.subclasses # => [B]

class A < B; end
C.subclasses # => [B]

class D < C; end
C.subclasses # => [B, D]
```

The order in which these classes are returned is unspecified.

NOTE: Defined in `active_support/core_ext/class/subclasses.rb`.

#### `descendants`

The `descendants` method returns all classes that are `<` than its receiver:

```ruby
class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]
```

The order in which these classes are returned is unspecified.

NOTE: Defined in `active_support/core_ext/class/subclasses.rb`.

Extensions to `String`
----------------------

### Output Safety

#### Motivation

Inserting data into HTML templates needs extra care. For example, you can't just interpolate `@review.title` verbatim into an HTML page. For one thing, if the review title is "Flanagan & Matz rules!" the output won't be well-formed because an ampersand has to be escaped as "&amp;amp;". What's more, depending on the application, that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the [Security guide](security.html#cross-site-scripting-xss) for further information about the risks.

#### Safe Strings

Active Support has the concept of <i>(html) safe</i> strings. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be <i>unsafe</i> by default:

```ruby
"".html_safe? # => false
```

You can obtain a safe string from a given one with the `html_safe` method:

```ruby
s = "".html_safe
s.html_safe? # => true
```

It is important to understand that `html_safe` performs no escaping whatsoever, it is just an assertion:

```ruby
s = "<script>...</script>".html_safe
s.html_safe? # => true
s            # => "<script>...</script>"
```

It is your responsibility to ensure calling `html_safe` on a particular string is fine.

If you append onto a safe string, either in-place with `concat`/`<<`, or with `+`, the result is a safe string. Unsafe arguments are escaped:

```ruby
"".html_safe + "<" # => "&lt;"
```

Safe arguments are directly appended:

```ruby
"".html_safe + "<".html_safe # => "<"
```

These methods should not be used in ordinary views. Unsafe values are automatically escaped:

```erb
<%= @review.title %> <%# fine, escaped if needed %>
```

To insert something verbatim use the `raw` helper rather than calling `html_safe`:

```erb
<%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>
```

or, equivalently, use `<%==`:

```erb
<%== @cms.current_template %> <%# inserts @cms.current_template as is %>
```

The `raw` helper calls `html_safe` for you:

```ruby
def raw(stringish)
  stringish.to_s.html_safe
end
```

NOTE: Defined in `active_support/core_ext/string/output_safety.rb`.

#### Transformation

As a rule of thumb, except perhaps for concatenation as explained above, any method that may change a string gives you an unsafe string. These are `downcase`, `gsub`, `strip`, `chomp`, `underscore`, etc.

In the case of in-place transformations like `gsub!` the receiver itself becomes unsafe.

INFO: The safety bit is lost always, no matter whether the transformation actually changed something.

#### Conversion and Coercion

Calling `to_s` on a safe string returns a safe string, but coercion with `to_str` returns an unsafe string.

#### Copying

Calling `dup` or `clone` on safe strings yields safe strings.

### `squish`

The method `squish` strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

```ruby
" \n  foo\n\r \t bar \n".squish # => "foo bar"
```

There's also the destructive version `String#squish!`.

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `truncate`

The method `truncate` returns a copy of its receiver truncated after a given `length`:

```ruby
"Oh dear! Oh dear! I shall be late!".truncate(20)
# => "Oh dear! Oh dear!..."
```

Ellipsis can be customized with the `:omission` option:

```ruby
"Oh dear! Oh dear! I shall be late!".truncate(20, omission: '&hellip;')
# => "Oh dear! Oh &hellip;"
```

Note in particular that truncation takes into account the length of the omission string.

Pass a `:separator` to truncate the string at a natural break:

```ruby
"Oh dear! Oh dear! I shall be late!".truncate(18)
# => "Oh dear! Oh dea..."
"Oh dear! Oh dear! I shall be late!".truncate(18, separator: ' ')
# => "Oh dear! Oh..."
```

The option `:separator` can be a regexp:

```ruby
"Oh dear! Oh dear! I shall be late!".truncate(18, separator: /\s/)
# => "Oh dear! Oh..."
```

In above examples "dear" gets cut first, but then `:separator` prevents it.

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `inquiry`

The `inquiry` method converts a string into a `StringInquirer` object making equality checks prettier.

```ruby
"production".inquiry.production? # => true
"active".inquiry.inactive?       # => false
```

### `starts_with?` and `ends_with?`

Active Support defines 3rd person aliases of `String#start_with?` and `String#end_with?`:

```ruby
"foo".starts_with?("f") # => true
"foo".ends_with?("o")   # => true
```

NOTE: Defined in `active_support/core_ext/string/starts_ends_with.rb`.

### `strip_heredoc`

The method `strip_heredoc` strips indentation in heredocs.

For example in

```ruby
if options[:usage]
  puts <<-USAGE.strip_heredoc
    This command does such and such.

    Supported options are:
      -h         This message
      ...
  USAGE
end
```

the user would see the usage message aligned against the left margin.

Technically, it looks for the least indented line in the whole string, and removes
that amount of leading whitespace.

NOTE: Defined in `active_support/core_ext/string/strip.rb`.

### `indent`

Indents the lines in the receiver:

```ruby
<<EOS.indent(2)
def some_method
  some_code
end
EOS
# =>
  def some_method
    some_code
  end
```

The second argument, `indent_string`, specifies which indent string to use. The default is `nil`, which tells the method to make an educated guess peeking at the first indented line, and fallback to a space if there is none.

```ruby
"  foo".indent(2)        # => "    foo"
"foo\n\t\tbar".indent(2) # => "\t\tfoo\n\t\t\t\tbar"
"foo".indent(2, "\t")    # => "\t\tfoo"
```

While `indent_string` is tipically one space or tab, it may be any string.

The third argument, `indent_empty_lines`, is a flag that says whether empty lines should be indented. Default is false.

```ruby
"foo\n\nbar".indent(2)            # => "  foo\n\n  bar"
"foo\n\nbar".indent(2, nil, true) # => "  foo\n  \n  bar"
```

The `indent!` method performs indentation in-place.

### Access

#### `at(position)`

Returns the character of the string at position `position`:

```ruby
"hello".at(0)  # => "h"
"hello".at(4)  # => "o"
"hello".at(-1) # => "o"
"hello".at(10) # => nil
```

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `from(position)`

Returns the substring of the string starting at position `position`:

```ruby
"hello".from(0)  # => "hello"
"hello".from(2)  # => "llo"
"hello".from(-2) # => "lo"
"hello".from(10) # => "" if < 1.9, nil in 1.9
```

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `to(position)`

Returns the substring of the string up to position `position`:

```ruby
"hello".to(0)  # => "h"
"hello".to(2)  # => "hel"
"hello".to(-2) # => "hell"
"hello".to(10) # => "hello"
```

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `first(limit = 1)`

The call `str.first(n)` is equivalent to `str.to(n-1)` if `n` > 0, and returns an empty string for `n` == 0.

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `last(limit = 1)`

The call `str.last(n)` is equivalent to `str.from(-n)` if `n` > 0, and returns an empty string for `n` == 0.

NOTE: Defined in `active_support/core_ext/string/access.rb`.

### Inflections

#### `pluralize`

The method `pluralize` returns the plural of its receiver:

```ruby
"table".pluralize     # => "tables"
"ruby".pluralize      # => "rubies"
"equipment".pluralize # => "equipment"
```

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in `config/initializers/inflections.rb`. That file is generated by the `rails` command and has instructions in comments.

`pluralize` can also take an optional `count` parameter.  If `count == 1` the singular form will be returned.  For any other value of `count` the plural form will be returned:

```ruby
"dude".pluralize(0) # => "dudes"
"dude".pluralize(1) # => "dude"
"dude".pluralize(2) # => "dudes"
```

Active Record uses this method to compute the default table name that corresponds to a model:

```ruby
# active_record/model_schema.rb
def undecorated_table_name(class_name = base_class.name)
  table_name = class_name.to_s.demodulize.underscore
  pluralize_table_names ? table_name.pluralize : table_name
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `singularize`

The inverse of `pluralize`:

```ruby
"tables".singularize    # => "table"
"rubies".singularize    # => "ruby"
"equipment".singularize # => "equipment"
```

Associations compute the name of the corresponding default associated class using this method:

```ruby
# active_record/reflection.rb
def derive_class_name
  class_name = name.to_s.camelize
  class_name = class_name.singularize if collection?
  class_name
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `camelize`

The method `camelize` returns its receiver in camel case:

```ruby
"product".camelize    # => "Product"
"admin_user".camelize # => "AdminUser"
```

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

```ruby
"backoffice/session".camelize # => "Backoffice::Session"
```

For example, Action Pack uses this method to load the class that provides a certain session store:

```ruby
# action_controller/metal/session_management.rb
def session_store=(store)
  @@session_store = store.is_a?(Symbol) ?
    ActionDispatch::Session.const_get(store.to_s.camelize) :
    store
end
```

`camelize` accepts an optional argument, it can be `:upper` (default), or `:lower`. With the latter the first letter becomes lowercase:

```ruby
"visual_effect".camelize(:lower) # => "visualEffect"
```

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

INFO: As a rule of thumb you can think of `camelize` as the inverse of `underscore`, though there are cases where that does not hold: `"SSLError".underscore.camelize` gives back `"SslError"`. To support cases such as this, Active Support allows you to specify acronyms in `config/initializers/inflections.rb`:

```ruby
ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym 'SSL'
end

"SSLError".underscore.camelize #=> "SSLError"
```

`camelize` is aliased to `camelcase`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `underscore`

The method `underscore` goes the other way around, from camel case to paths:

```ruby
"Product".underscore   # => "product"
"AdminUser".underscore # => "admin_user"
```

Also converts "::" back to "/":

```ruby
"Backoffice::Session".underscore # => "backoffice/session"
```

and understands strings that start with lowercase:

```ruby
"visualEffect".underscore # => "visual_effect"
```

`underscore` accepts no argument though.

Rails class and module autoloading uses `underscore` to infer the relative path without extension of a file that would define a given missing constant:

```ruby
# active_support/dependencies.rb
def load_missing_constant(from_mod, const_name)
  ...
  qualified_name = qualified_name_for from_mod, const_name
  path_suffix = qualified_name.underscore
  ...
end
```

INFO: As a rule of thumb you can think of `underscore` as the inverse of `camelize`, though there are cases where that does not hold. For example, `"SSLError".underscore.camelize` gives back `"SslError"`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `titleize`

The method `titleize` capitalizes the words in the receiver:

```ruby
"alice in wonderland".titleize # => "Alice In Wonderland"
"fermat's enigma".titleize     # => "Fermat's Enigma"
```

`titleize` is aliased to `titlecase`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `dasherize`

The method `dasherize` replaces the underscores in the receiver with dashes:

```ruby
"name".dasherize         # => "name"
"contact_data".dasherize # => "contact-data"
```

The XML serializer of models uses this method to dasherize node names:

```ruby
# active_model/serializers/xml.rb
def reformat_name(name)
  name = name.camelize if camelize?
  dasherize? ? name.dasherize : name
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `demodulize`

Given a string with a qualified constant name, `demodulize` returns the very constant name, that is, the rightmost part of it:

```ruby
"Product".demodulize                        # => "Product"
"Backoffice::UsersController".demodulize    # => "UsersController"
"Admin::Hotel::ReservationUtils".demodulize # => "ReservationUtils"
```

Active Record for example uses this method to compute the name of a counter cache column:

```ruby
# active_record/reflection.rb
def counter_cache_column
  if options[:counter_cache] == true
    "#{active_record.name.demodulize.underscore.pluralize}_count"
  elsif options[:counter_cache]
    options[:counter_cache]
  end
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `deconstantize`

Given a string with a qualified constant reference expression, `deconstantize` removes the rightmost segment, generally leaving the name of the constant's container:

```ruby
"Product".deconstantize                        # => ""
"Backoffice::UsersController".deconstantize    # => "Backoffice"
"Admin::Hotel::ReservationUtils".deconstantize # => "Admin::Hotel"
```

Active Support for example uses this method in `Module#qualified_const_set`:

```ruby
def qualified_const_set(path, value)
  QualifiedConstUtils.raise_if_absolute(path)

  const_name = path.demodulize
  mod_name = path.deconstantize
  mod = mod_name.empty? ? self : qualified_const_get(mod_name)
  mod.const_set(const_name, value)
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `parameterize`

The method `parameterize` normalizes its receiver in a way that can be used in pretty URLs.

```ruby
"John Smith".parameterize # => "john-smith"
"Kurt Gödel".parameterize # => "kurt-godel"
```

In fact, the result string is wrapped in an instance of `ActiveSupport::Multibyte::Chars`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `tableize`

The method `tableize` is `underscore` followed by `pluralize`.

```ruby
"Person".tableize      # => "people"
"Invoice".tableize     # => "invoices"
"InvoiceLine".tableize # => "invoice_lines"
```

As a rule of thumb, `tableize` returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight `tableize` indeed, because it also demodulizes the class name and checks a few options that may affect the returned string.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `classify`

The method `classify` is the inverse of `tableize`. It gives you the class name corresponding to a table name:

```ruby
"people".classify        # => "Person"
"invoices".classify      # => "Invoice"
"invoice_lines".classify # => "InvoiceLine"
```

The method understands qualified table names:

```ruby
"highrise_production.companies".classify # => "Company"
```

Note that `classify` returns a class name as a string. You can get the actual class object invoking `constantize` on it, explained next.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `constantize`

The method `constantize` resolves the constant reference expression in its receiver:

```ruby
"Fixnum".constantize # => Fixnum

module M
  X = 1
end
"M::X".constantize # => 1
```

If the string evaluates to no known constant, or its content is not even a valid constant name, `constantize` raises `NameError`.

Constant name resolution by `constantize` starts always at the top-level `Object` even if there is no leading "::".

```ruby
X = :in_Object
module M
  X = :in_M

  X                 # => :in_M
  "::X".constantize # => :in_Object
  "X".constantize   # => :in_Object (!)
end
```

So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using `constantize`:

```ruby
# action_mailer/test_case.rb
def determine_default_mailer(name)
  name.sub(/Test$/, '').constantize
rescue NameError => e
  raise NonInferrableMailerError.new(name)
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `humanize`

The method `humanize` gives you a sensible name for display out of an attribute name. To do so it replaces underscores with spaces, removes any "_id" suffix, and capitalizes the first word:

```ruby
"name".humanize           # => "Name"
"author_id".humanize      # => "Author"
"comments_count".humanize # => "Comments count"
```

The helper method `full_messages` uses `humanize` as a fallback to include attribute names:

```ruby
def full_messages
  full_messages = []

  each do |attribute, messages|
    ...
    attr_name = attribute.to_s.gsub('.', '_').humanize
    attr_name = @base.class.human_attribute_name(attribute, default: attr_name)
    ...
  end

  full_messages
end
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `foreign_key`

The method `foreign_key` gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds "_id":

```ruby
"User".foreign_key           # => "user_id"
"InvoiceLine".foreign_key    # => "invoice_line_id"
"Admin::Session".foreign_key # => "session_id"
```

Pass a false argument if you do not want the underscore in "_id":

```ruby
"User".foreign_key(false) # => "userid"
```

Associations use this method to infer foreign keys, for example `has_one` and `has_many` do this:

```ruby
# active_record/associations.rb
foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key
```

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

### Conversions

#### `to_date`, `to_time`, `to_datetime`

The methods `to_date`, `to_time`, and `to_datetime` are basically convenience wrappers around `Date._parse`:

```ruby
"2010-07-27".to_date              # => Tue, 27 Jul 2010
"2010-07-27 23:37:00".to_time     # => Tue Jul 27 23:37:00 UTC 2010
"2010-07-27 23:37:00".to_datetime # => Tue, 27 Jul 2010 23:37:00 +0000
```

`to_time` receives an optional argument `:utc` or `:local`, to indicate which time zone you want the time in:

```ruby
"2010-07-27 23:42:00".to_time(:utc)   # => Tue Jul 27 23:42:00 UTC 2010
"2010-07-27 23:42:00".to_time(:local) # => Tue Jul 27 23:42:00 +0200 2010
```

Default is `:utc`.

Please refer to the documentation of `Date._parse` for further details.

INFO: The three of them return `nil` for blank receivers.

NOTE: Defined in `active_support/core_ext/string/conversions.rb`.

Extensions to `Numeric`
-----------------------

### Bytes

All numbers respond to these methods:

```ruby
bytes
kilobytes
megabytes
gigabytes
terabytes
petabytes
exabytes
```

They return the corresponding amount of bytes, using a conversion factor of 1024:

```ruby
2.kilobytes   # => 2048
3.megabytes   # => 3145728
3.5.gigabytes # => 3758096384
-4.exabytes   # => -4611686018427387904
```

Singular forms are aliased so you are able to say:

```ruby
1.megabyte # => 1048576
```

NOTE: Defined in `active_support/core_ext/numeric/bytes.rb`.

### Time

Enables the use of time calculations and declarations, like `45.minutes + 2.hours + 4.years`.

These methods use Time#advance for precise date calculations when using from_now, ago, etc.
as well as adding or subtracting their results from a Time object. For example:

```ruby
# equivalent to Time.current.advance(months: 1)
1.month.from_now

# equivalent to Time.current.advance(years: 2)
2.years.from_now

# equivalent to Time.current.advance(months: 4, years: 5)
(4.months + 5.years).from_now
```

While these methods provide precise calculation when used as in the examples above, care
should be taken to note that this is not true if the result of `months', `years', etc is
converted before use:

```ruby
# equivalent to 30.days.to_i.from_now
1.month.to_i.from_now

# equivalent to 365.25.days.to_f.from_now
1.year.to_f.from_now
```

In such cases, Ruby's core [Date](http://ruby-doc.org/stdlib/libdoc/date/rdoc/Date.html) and
[Time](http://ruby-doc.org/stdlib/libdoc/time/rdoc/Time.html) should be used for precision
date and time arithmetic.

NOTE: Defined in `active_support/core_ext/numeric/time.rb`.

### Formatting

Enables the formatting of numbers in a variety of ways.

Produce a string representation of a number as a telephone number:

```ruby
5551234.to_s(:phone)
# => 555-1234
1235551234.to_s(:phone)
# => 123-555-1234
1235551234.to_s(:phone, area_code: true)
# => (123) 555-1234
1235551234.to_s(:phone, delimiter: " ")
# => 123 555 1234
1235551234.to_s(:phone, area_code: true, extension: 555)
# => (123) 555-1234 x 555
1235551234.to_s(:phone, country_code: 1)
# => +1-123-555-1234
```

Produce a string representation of a number as currency:

```ruby
1234567890.50.to_s(:currency)                 # => $1,234,567,890.50
1234567890.506.to_s(:currency)                # => $1,234,567,890.51
1234567890.506.to_s(:currency, precision: 3)  # => $1,234,567,890.506
```

Produce a string representation of a number as a percentage:

```ruby
100.to_s(:percentage)
# => 100.000%
100.to_s(:percentage, precision: 0)
# => 100%
1000.to_s(:percentage, delimiter: '.', separator: ',')
# => 1.000,000%
302.24398923423.to_s(:percentage, precision: 5)
# => 302.24399%
```

Produce a string representation of a number in delimited form:

```ruby
12345678.to_s(:delimited)                     # => 12,345,678
12345678.05.to_s(:delimited)                  # => 12,345,678.05
12345678.to_s(:delimited, delimiter: ".")     # => 12.345.678
12345678.to_s(:delimited, delimiter: ",")     # => 12,345,678
12345678.05.to_s(:delimited, separator: " ")  # => 12,345,678 05
```

Produce a string representation of a number rounded to a precision:

```ruby
111.2345.to_s(:rounded)                     # => 111.235
111.2345.to_s(:rounded, precision: 2)       # => 111.23
13.to_s(:rounded, precision: 5)             # => 13.00000
389.32314.to_s(:rounded, precision: 0)      # => 389
111.2345.to_s(:rounded, significant: true)  # => 111
```

Produce a string representation of a number as a human-readable number of bytes:

```ruby
123.to_s(:human_size)            # => 123 Bytes
1234.to_s(:human_size)           # => 1.21 KB
12345.to_s(:human_size)          # => 12.1 KB
1234567.to_s(:human_size)        # => 1.18 MB
1234567890.to_s(:human_size)     # => 1.15 GB
1234567890123.to_s(:human_size)  # => 1.12 TB
```

Produce a string representation of a number in human-readable words:

```ruby
123.to_s(:human)               # => "123"
1234.to_s(:human)              # => "1.23 Thousand"
12345.to_s(:human)             # => "12.3 Thousand"
1234567.to_s(:human)           # => "1.23 Million"
1234567890.to_s(:human)        # => "1.23 Billion"
1234567890123.to_s(:human)     # => "1.23 Trillion"
1234567890123456.to_s(:human)  # => "1.23 Quadrillion"
```

NOTE: Defined in `active_support/core_ext/numeric/formatting.rb`.

Extensions to `Integer`
-----------------------

### `multiple_of?`

The method `multiple_of?` tests whether an integer is multiple of the argument:

```ruby
2.multiple_of?(1) # => true
1.multiple_of?(2) # => false
```

NOTE: Defined in `active_support/core_ext/integer/multiple.rb`.

### `ordinal`

The method `ordinal` returns the ordinal suffix string corresponding to the receiver integer:

```ruby
1.ordinal    # => "st"
2.ordinal    # => "nd"
53.ordinal   # => "rd"
2009.ordinal # => "th"
-21.ordinal  # => "st"
-134.ordinal # => "th"
```

NOTE: Defined in `active_support/core_ext/integer/inflections.rb`.

### `ordinalize`

The method `ordinalize` returns the ordinal string corresponding to the receiver integer. In comparison, note that the `ordinal` method returns **only** the suffix string.

```ruby
1.ordinalize    # => "1st"
2.ordinalize    # => "2nd"
53.ordinalize   # => "53rd"
2009.ordinalize # => "2009th"
-21.ordinalize  # => "-21st"
-134.ordinalize # => "-134th"
```

NOTE: Defined in `active_support/core_ext/integer/inflections.rb`.

Extensions to `BigDecimal`
--------------------------

...

Extensions to `Enumerable`
--------------------------

### `sum`

The method `sum` adds the elements of an enumerable:

```ruby
[1, 2, 3].sum # => 6
(1..100).sum  # => 5050
```

Addition only assumes the elements respond to `+`:

```ruby
[[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
%w(foo bar baz).sum             # => "foobarbaz"
{a: 1, b: 2, c: 3}.sum # => [:b, 2, :c, 3, :a, 1]
```

The sum of an empty collection is zero by default, but this is customizable:

```ruby
[].sum    # => 0
[].sum(1) # => 1
```

If a block is given, `sum` becomes an iterator that yields the elements of the collection and sums the returned values:

```ruby
(1..5).sum {|n| n * 2 } # => 30
[2, 4, 6, 8, 10].sum    # => 30
```

The sum of an empty receiver can be customized in this form as well:

```ruby
[].sum(1) {|n| n**3} # => 1
```

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `index_by`

The method `index_by` generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:

```ruby
invoices.index_by(&:number)
# => {'2009-032' => <Invoice ...>, '2009-008' => <Invoice ...>, ...}
```

WARNING. Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `many?`

The method `many?` is shorthand for `collection.size > 1`:

```erb
<% if pages.many? %>
  <%= pagination_links %>
<% end %>
```

If an optional block is given, `many?` only takes into account those elements that return true:

```ruby
@see_more = videos.many? {|video| video.category == params[:category]}
```

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `exclude?`

The predicate `exclude?` tests whether a given object does **not** belong to the collection. It is the negation of the built-in `include?`:

```ruby
to_visit << node if visited.exclude?(node)
```

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

Extensions to `Array`
---------------------

### Accessing

Active Support augments the API of arrays to ease certain ways of accessing them. For example, `to` returns the subarray of elements up to the one at the passed index:

```ruby
%w(a b c d).to(2) # => %w(a b c)
[].to(7)          # => []
```

Similarly, `from` returns the tail from the element at the passed index to the end. If the index is greater than the length of the array, it returns an empty array.

```ruby
%w(a b c d).from(2)  # => %w(c d)
%w(a b c d).from(10) # => []
[].from(0)           # => []
```

The methods `second`, `third`, `fourth`, and `fifth` return the corresponding element (`first` is built-in). Thanks to social wisdom and positive constructiveness all around, `forty_two` is also available.

```ruby
%w(a b c d).third # => c
%w(a b c d).fifth # => nil
```

NOTE: Defined in `active_support/core_ext/array/access.rb`.

### Adding Elements

#### `prepend`

This method is an alias of `Array#unshift`.

```ruby
%w(a b c d).prepend('e')  # => %w(e a b c d)
[].prepend(10)            # => [10]
```

NOTE: Defined in `active_support/core_ext/array/prepend_and_append.rb`.

#### `append`

This method is an alias of `Array#<<`.

```ruby
%w(a b c d).append('e')  # => %w(a b c d e)
[].append([1,2])         # => [[1,2]]
```

NOTE: Defined in `active_support/core_ext/array/prepend_and_append.rb`.

### Options Extraction

When the last argument in a method call is a hash, except perhaps for a `&block` argument, Ruby allows you to omit the brackets:

```ruby
User.exists?(email: params[:email])
```

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses `*` in its declaration, however, such an options hash ends up being an item of the array of arguments, where it loses its role.

In those cases, you may give an options hash a distinguished treatment with `extract_options!`. This method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise it returns an empty hash.

Let's see for example the definition of the `caches_action` controller macro:

```ruby
def caches_action(*actions)
  return unless cache_configured?
  options = actions.extract_options!
  ...
end
```

This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to `extract_options!` you obtain the options hash and remove it from `actions` in a simple and explicit way.

NOTE: Defined in `active_support/core_ext/array/extract_options.rb`.

### Conversions

#### `to_sentence`

The method `to_sentence` turns an array into a string containing a sentence that enumerates its items:

```ruby
%w().to_sentence                # => ""
%w(Earth).to_sentence           # => "Earth"
%w(Earth Wind).to_sentence      # => "Earth and Wind"
%w(Earth Wind Fire).to_sentence # => "Earth, Wind, and Fire"
```

This method accepts three options:

* `:two_words_connector`: What is used for arrays of length 2. Default is " and ".
* `:words_connector`: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
* `:last_word_connector`: What is used to join the last items of an array with 3 or more elements. Default is ", and ".

The defaults for these options can be localised, their keys are:

| Option                 | I18n key                            |
| ---------------------- | ----------------------------------- |
| `:two_words_connector` | `support.array.two_words_connector` |
| `:words_connector`     | `support.array.words_connector`     |
| `:last_word_connector` | `support.array.last_word_connector` |

Options `:connector` and `:skip_last_comma` are deprecated.

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

#### `to_formatted_s`

The method `to_formatted_s` acts like `to_s` by default.

If the array contains items that respond to `id`, however, it may be passed the symbol `:db` as argument. That's typically used with collections of ARs. Returned strings are:

```ruby
[].to_formatted_s(:db)            # => "null"
[user].to_formatted_s(:db)        # => "8456"
invoice.lines.to_formatted_s(:db) # => "23,567,556,12"
```

Integers in the example above are supposed to come from the respective calls to `id`.

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

#### `to_xml`

The method `to_xml` returns a string containing an XML representation of its receiver:

```ruby
Contributor.limit(2).order(:rank).to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors type="array">
#   <contributor>
#     <id type="integer">4356</id>
#     <name>Jeremy Kemper</name>
#     <rank type="integer">1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id type="integer">4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank type="integer">2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>
```

To do so it sends `to_xml` to every item in turn, and collects the results under a root node. All items must respond to `to_xml`, an exception is raised otherwise.

By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with `is_a?`) and they are not hashes. In the example above that's "contributors".

If there's any element that does not belong to the type of the first one the root node becomes "objects":

```ruby
[Contributor.first, Commit.first].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <id type="integer">4583</id>
#     <name>Aaron Batalion</name>
#     <rank type="integer">53</rank>
#     <url-id>aaron-batalion</url-id>
#   </object>
#   <object>
#     <author>Joshua Peek</author>
#     <authored-timestamp type="datetime">2009-09-02T16:44:36Z</authored-timestamp>
#     <branch>origin/master</branch>
#     <committed-timestamp type="datetime">2009-09-02T16:44:36Z</committed-timestamp>
#     <committer>Joshua Peek</committer>
#     <git-show nil="true"></git-show>
#     <id type="integer">190316</id>
#     <imported-from-svn type="boolean">false</imported-from-svn>
#     <message>Kill AMo observing wrap_with_notifications since ARes was only using it</message>
#     <sha1>723a47bfb3708f968821bc969a9a3fc873a3ed58</sha1>
#   </object>
# </objects>
```

If the receiver is an array of hashes the root element is by default also "objects":

```ruby
[{a: 1, b: 2}, {c: 3}].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <b type="integer">2</b>
#     <a type="integer">1</a>
#   </object>
#   <object>
#     <c type="integer">3</c>
#   </object>
# </objects>
```

WARNING. If the collection is empty the root element is by default "nil-classes". That's a gotcha, for example the root element of the list of contributors above would not be "contributors" if the collection was empty, but "nil-classes". You may use the `:root` option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we've seen "contributor" and "object". The option `:children` allows you to set these node names.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder via the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder:

```ruby
Contributor.limit(2).order(:rank).to_xml(skip_types: true)
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors>
#   <contributor>
#     <id>4356</id>
#     <name>Jeremy Kemper</name>
#     <rank>1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id>4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank>2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>
```

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

### Wrapping

The method `Array.wrap` wraps its argument in an array unless it is already an array (or array-like).

Specifically:

* If the argument is `nil` an empty list is returned.
* Otherwise, if the argument responds to `to_ary` it is invoked, and if the value of `to_ary` is not `nil`, it is returned.
* Otherwise, an array with the argument as its single element is returned.

```ruby
Array.wrap(nil)       # => []
Array.wrap([1, 2, 3]) # => [1, 2, 3]
Array.wrap(0)         # => [0]
```

This method is similar in purpose to `Kernel#Array`, but there are some differences:

* If the argument responds to `to_ary` the method is invoked. `Kernel#Array` moves on to try `to_a` if the returned value is `nil`, but `Array.wrap` returns `nil` right away.
* If the returned value from `to_ary` is neither `nil` nor an `Array` object, `Kernel#Array` raises an exception, while `Array.wrap` does not, it just returns the value.
* It does not call `to_a` on the argument, though special-cases `nil` to return an empty array.

The last point is particularly worth comparing for some enumerables:

```ruby
Array.wrap(foo: :bar) # => [{:foo=>:bar}]
Array(foo: :bar)      # => [[:foo, :bar]]
```

There's also a related idiom that uses the splat operator:

```ruby
[*object]
```

which in Ruby 1.8 returns `[nil]` for `nil`, and calls to `Array(object)` otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)

Thus, in this case the behavior is different for `nil`, and the differences with `Kernel#Array` explained above apply to the rest of `object`s.

NOTE: Defined in `active_support/core_ext/array/wrap.rb`.

### Duplicating

The method `Array.deep_dup` duplicates itself and all objects inside recursively with ActiveSupport method `Object#deep_dup`. It works like `Array#map` with sending `deep_dup` method to each object inside.

```ruby
array = [1, [2, 3]]
dup = array.deep_dup
dup[1][2] = 4
array[1][2] == nil   # => true
```

NOTE: Defined in `active_support/core_ext/array/deep_dup.rb`.

### Grouping

#### `in_groups_of(number, fill_with = nil)`

The method `in_groups_of` splits an array into consecutive groups of a certain size. It returns an array with the groups:

```ruby
[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]
```

or yields them in turn if a block is passed:

```html+erb
<% sample.in_groups_of(3) do |a, b, c| %>
  <tr>
    <td><%= a %></td>
    <td><%= b %></td>
    <td><%= c %></td>
  </tr>
<% end %>
```

The first example shows `in_groups_of` fills the last group with as many `nil` elements as needed to have the requested size. You can change this padding value using the second optional argument:

```ruby
[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]
```

And you can tell the method not to fill the last group passing `false`:

```ruby
[1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]
```

As a consequence `false` can't be a used as a padding value.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

#### `in_groups(number, fill_with = nil)`

The method `in_groups` splits an array into a certain number of groups. The method returns an array with the groups:

```ruby
%w(1 2 3 4 5 6 7).in_groups(3)
# => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]
```

or yields them in turn if a block is passed:

```ruby
%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
["1", "2", "3"]
["4", "5", nil]
["6", "7", nil]
```

The examples above show that `in_groups` fills some groups with a trailing `nil` element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

```ruby
%w(1 2 3 4 5 6 7).in_groups(3, "0")
# => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]
```

And you can tell the method not to fill the smaller groups passing `false`:

```ruby
%w(1 2 3 4 5 6 7).in_groups(3, false)
# => [["1", "2", "3"], ["4", "5"], ["6", "7"]]
```

As a consequence `false` can't be a used as a padding value.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

#### `split(value = nil)`

The method `split` divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

```ruby
(-5..5).to_a.split { |i| i.multiple_of?(4) }
# => [[-5], [-3, -2, -1], [1, 2, 3], [5]]
```

Otherwise, the value received as argument, which defaults to `nil`, is the separator:

```ruby
[0, 1, -5, 1, 1, "foo", "bar"].split(1)
# => [[0], [-5], [], ["foo", "bar"]]
```

TIP: Observe in the previous example that consecutive separators result in empty arrays.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

Extensions to `Hash`
--------------------

### Conversions

#### `to_xml`

The method `to_xml` returns a string containing an XML representation of its receiver:

```ruby
{"foo" => 1, "bar" => 2}.to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <hash>
#   <foo type="integer">1</foo>
#   <bar type="integer">2</bar>
# </hash>
```

To do so, the method loops over the pairs and builds nodes that depend on the _values_. Given a pair `key`, `value`:

* If `value` is a hash there's a recursive call with `key` as `:root`.

* If `value` is an array there's a recursive call with `key` as `:root`, and `key` singularized as `:children`.

* If `value` is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the `options` hash as first argument with `key` as `:root`, and `key` singularized as second argument. Its return value becomes a new node.

* If `value` responds to `to_xml` the method is invoked with `key` as `:root`.

* Otherwise, a node with `key` as tag is created with a string representation of `value` as text node. If `value` is `nil` an attribute "nil" set to "true" is added. Unless the option `:skip_types` exists and is true, an attribute "type" is added as well according to the following mapping:

```ruby
XML_TYPE_NAMES = {
  "Symbol"     => "symbol",
  "Fixnum"     => "integer",
  "Bignum"     => "integer",
  "BigDecimal" => "decimal",
  "Float"      => "float",
  "TrueClass"  => "boolean",
  "FalseClass" => "boolean",
  "Date"       => "date",
  "DateTime"   => "datetime",
  "Time"       => "datetime"
}
```

By default the root node is "hash", but that's configurable via the `:root` option.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder with the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder.

NOTE: Defined in `active_support/core_ext/hash/conversions.rb`.

### Merging

Ruby has a built-in method `Hash#merge` that merges two hashes:

```ruby
{a: 1, b: 1}.merge(a: 0, c: 2)
# => {:a=>0, :b=>1, :c=>2}
```

Active Support defines a few more ways of merging hashes that may be convenient.

#### `reverse_merge` and `reverse_merge!`

In case of collision the key in the hash of the argument wins in `merge`. You can support option hashes with default values in a compact way with this idiom:

```ruby
options = {length: 30, omission: "..."}.merge(options)
```

Active Support defines `reverse_merge` in case you prefer this alternative notation:

```ruby
options = options.reverse_merge(length: 30, omission: "...")
```

And a bang version `reverse_merge!` that performs the merge in place:

```ruby
options.reverse_merge!(length: 30, omission: "...")
```

WARNING. Take into account that `reverse_merge!` may change the hash in the caller, which may or may not be a good idea.

NOTE: Defined in `active_support/core_ext/hash/reverse_merge.rb`.

#### `reverse_update`

The method `reverse_update` is an alias for `reverse_merge!`, explained above.

WARNING. Note that `reverse_update` has no bang.

NOTE: Defined in `active_support/core_ext/hash/reverse_merge.rb`.

#### `deep_merge` and `deep_merge!`

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines `Hash#deep_merge`. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their _merge_ becomes the value in the resulting hash:

```ruby
{a: {b: 1}}.deep_merge(a: {c: 2})
# => {:a=>{:b=>1, :c=>2}}
```

The method `deep_merge!` performs a deep merge in place.

NOTE: Defined in `active_support/core_ext/hash/deep_merge.rb`.

### Deep duplicating

The method `Hash.deep_dup` duplicates itself and all keys and values inside recursively with ActiveSupport method `Object#deep_dup`. It works like `Enumerator#each_with_object` with sending `deep_dup` method to each pair inside.

```ruby
hash = { a: 1, b: { c: 2, d: [3, 4] } }

dup = hash.deep_dup
dup[:b][:e] = 5
dup[:b][:d] << 5

hash[:b][:e] == nil      # => true
hash[:b][:d] == [3, 4]   # => true
```

NOTE: Defined in `active_support/core_ext/hash/deep_dup.rb`.

### Diffing

The method `diff` returns a hash that represents a diff of the receiver and the argument with the following logic:

* Pairs `key`, `value` that exist in both hashes do not belong to the diff hash.

* If both hashes have `key`, but with different values, the pair in the receiver wins.

* The rest is just merged.

```ruby
{a: 1}.diff(a: 1)
# => {}, first rule

{a: 1}.diff(a: 2)
# => {:a=>1}, second rule

{a: 1}.diff(b: 2)
# => {:a=>1, :b=>2}, third rule

{a: 1, b: 2, c: 3}.diff(b: 1, c: 3, d: 4)
# => {:a=>1, :b=>2, :d=>4}, all rules

{}.diff({})        # => {}
{a: 1}.diff({})    # => {:a=>1}
{}.diff(a: 1)      # => {:a=>1}
```

An important property of this diff hash is that you can retrieve the original hash by applying `diff` twice:

```ruby
hash.diff(hash2).diff(hash2) == hash
```

Diffing hashes may be useful for error messages related to expected option hashes for example.

NOTE: Defined in `active_support/core_ext/hash/diff.rb`.

### Working with Keys

#### `except` and `except!`

The method `except` returns a hash with the keys in the argument list removed, if present:

```ruby
{a: 1, b: 2}.except(:a) # => {:b=>2}
```

If the receiver responds to `convert_key`, the method is called on each of the arguments. This allows `except` to play nice with hashes with indifferent access for instance:

```ruby
{a: 1}.with_indifferent_access.except(:a)  # => {}
{a: 1}.with_indifferent_access.except("a") # => {}
```

There's also the bang variant `except!` that removes keys in the very receiver.

NOTE: Defined in `active_support/core_ext/hash/except.rb`.

#### `transform_keys` and `transform_keys!`

The method `transform_keys` accepts a block and returns a hash that has applied the block operations to each of the keys in the receiver:

```ruby
{nil => nil, 1 => 1, a: :a}.transform_keys{ |key| key.to_s.upcase }
# => {"" => nil, "A" => :a, "1" => 1}
```

The result in case of collision is undefined:

```ruby
{"a" => 1, a: 2}.transform_keys{ |key| key.to_s.upcase }
# => {"A" => 2}, in my test, can't rely on this result though
```

This method may be useful for example to build specialized conversions. For instance `stringify_keys` and `symbolize_keys` use `transform_keys` to perform their key conversions:

```ruby
def stringify_keys
  transform_keys{ |key| key.to_s }
end
...
def symbolize_keys
  transform_keys{ |key| key.to_sym rescue key }
end
```

There's also the bang variant `transform_keys!` that applies the block operations to keys in the very receiver.

Besides that, one can use `deep_transform_keys` and `deep_transform_keys!` to perform the block operation on all the keys in the given hash and all the hashes nested into it. An example of the result is:

```ruby
{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_transform_keys{ |key| key.to_s.upcase }
# => {""=>nil, "1"=>1, "NESTED"=>{"A"=>3, "5"=>5}}
```

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `stringify_keys` and `stringify_keys!`

The method `stringify_keys` returns a hash that has a stringified version of the keys in the receiver. It does so by sending `to_s` to them:

```ruby
{nil => nil, 1 => 1, a: :a}.stringify_keys
# => {"" => nil, "a" => :a, "1" => 1}
```

The result in case of collision is undefined:

```ruby
{"a" => 1, a: 2}.stringify_keys
# => {"a" => 2}, in my test, can't rely on this result though
```

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionView::Helpers::FormHelper` defines:

```ruby
def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
  options = options.stringify_keys
  options["type"] = "checkbox"
  ...
end
```

The second line can safely access the "type" key, and let the user to pass either `:type` or "type".

There's also the bang variant `stringify_keys!` that stringifies keys in the very receiver.

Besides that, one can use `deep_stringify_keys` and `deep_stringify_keys!` to stringify all the keys in the given hash and all the hashes nested into it. An example of the result is:

```ruby
{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_stringify_keys
# => {""=>nil, "1"=>1, "nested"=>{"a"=>3, "5"=>5}}
```

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `symbolize_keys` and `symbolize_keys!`

The method `symbolize_keys` returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending `to_sym` to them:

```ruby
{nil => nil, 1 => 1, "a" => "a"}.symbolize_keys
# => {1=>1, nil=>nil, :a=>"a"}
```

WARNING. Note in the previous example only one key was symbolized.

The result in case of collision is undefined:

```ruby
{"a" => 1, a: 2}.symbolize_keys
# => {:a=>2}, in my test, can't rely on this result though
```

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionController::UrlRewriter` defines

```ruby
def rewrite_path(options)
  options = options.symbolize_keys
  options.update(options[:params].symbolize_keys) if options[:params]
  ...
end
```

The second line can safely access the `:params` key, and let the user to pass either `:params` or "params".

There's also the bang variant `symbolize_keys!` that symbolizes keys in the very receiver.

Besides that, one can use `deep_symbolize_keys` and `deep_symbolize_keys!` to symbolize all the keys in the given hash and all the hashes nested into it. An example of the result is:

```ruby
{nil => nil, 1 => 1, "nested" => {"a" => 3, 5 => 5}}.deep_symbolize_keys
# => {nil=>nil, 1=>1, nested:{a:3, 5=>5}}
```

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `to_options` and `to_options!`

The methods `to_options` and `to_options!` are respectively aliases of `symbolize_keys` and `symbolize_keys!`.

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `assert_valid_keys`

The method `assert_valid_keys` receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does `ArgumentError` is raised.

```ruby
{a: 1}.assert_valid_keys(:a)  # passes
{a: 1}.assert_valid_keys("a") # ArgumentError
```

Active Record does not accept unknown options when building associations, for example. It implements that control via `assert_valid_keys`.

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

### Slicing

Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

```ruby
{a: 1, b: 2, c: 3}.slice(:a, :c)
# => {:c=>3, :a=>1}

{a: 1, b: 2, c: 3}.slice(:b, :X)
# => {:b=>2} # non-existing keys are ignored
```

If the receiver responds to `convert_key` keys are normalized:

```ruby
{a: 1, b: 2}.with_indifferent_access.slice("a")
# => {:a=>1}
```

NOTE. Slicing may come in handy for sanitizing option hashes with a white list of keys.

There's also `slice!` which in addition to perform a slice in place returns what's removed:

```ruby
hash = {a: 1, b: 2}
rest = hash.slice!(:a) # => {:b=>2}
hash                   # => {:a=>1}
```

NOTE: Defined in `active_support/core_ext/hash/slice.rb`.

### Extracting

The method `extract!` removes and returns the key/value pairs matching the given keys.

```ruby
hash = {a: 1, b: 2}
rest = hash.extract!(:a) # => {:a=>1}
hash                     # => {:b=>2}
```

The method `extract!` returns the same subclass of Hash, that the receiver is.

```ruby
hash = {a: 1, b: 2}.with_indifferent_access
rest = hash.extract!(:a).class
# => ActiveSupport::HashWithIndifferentAccess
```

NOTE: Defined in `active_support/core_ext/hash/slice.rb`.

### Indifferent Access

The method `with_indifferent_access` returns an `ActiveSupport::HashWithIndifferentAccess` out of its receiver:

```ruby
{a: 1}.with_indifferent_access["a"] # => 1
```

NOTE: Defined in `active_support/core_ext/hash/indifferent_access.rb`.

Extensions to `Regexp`
----------------------

### `multiline?`

The method `multiline?` says whether a regexp has the `/m` flag set, that is, whether the dot matches newlines.

```ruby
%r{.}.multiline?  # => false
%r{.}m.multiline? # => true

Regexp.new('.').multiline?                    # => false
Regexp.new('.', Regexp::MULTILINE).multiline? # => true
```

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

```ruby
def assign_route_options(segments, defaults, requirements)
  ...
  if requirement.multiline?
    raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"
  end
  ...
end
```

NOTE: Defined in `active_support/core_ext/regexp.rb`.

Extensions to `Range`
---------------------

### `to_s`

Active Support extends the method `Range#to_s` so that it understands an optional format argument. As of this writing the only supported non-default format is `:db`:

```ruby
(Date.today..Date.tomorrow).to_s
# => "2009-10-25..2009-10-26"

(Date.today..Date.tomorrow).to_s(:db)
# => "BETWEEN '2009-10-25' AND '2009-10-26'"
```

As the example depicts, the `:db` format generates a `BETWEEN` SQL clause. That is used by Active Record in its support for range values in conditions.

NOTE: Defined in `active_support/core_ext/range/conversions.rb`.

### `include?`

The methods `Range#include?` and `Range#===` say whether some value falls between the ends of a given instance:

```ruby
(2..3).include?(Math::E) # => true
```

Active Support extends these methods so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

```ruby
(1..10).include?(3..7)  # => true
(1..10).include?(0..7)  # => false
(1..10).include?(3..11) # => false
(1...9).include?(3..9)  # => false

(1..10) === (3..7)  # => true
(1..10) === (0..7)  # => false
(1..10) === (3..11) # => false
(1...9) === (3..9)  # => false
```

NOTE: Defined in `active_support/core_ext/range/include_range.rb`.

### `overlaps?`

The method `Range#overlaps?` says whether any two given ranges have non-void intersection:

```ruby
(1..10).overlaps?(7..11)  # => true
(1..10).overlaps?(0..7)   # => true
(1..10).overlaps?(11..27) # => false
```

NOTE: Defined in `active_support/core_ext/range/overlaps.rb`.

Extensions to `Proc`
--------------------

### `bind`

As you surely know Ruby has an `UnboundMethod` class whose instances are methods that belong to the limbo of methods without a self. The method `Module#instance_method` returns an unbound method for example:

```ruby
Hash.instance_method(:delete) # => #<UnboundMethod: Hash#delete>
```

An unbound method is not callable as is, you need to bind it first to an object with `bind`:

```ruby
clear = Hash.instance_method(:clear)
clear.bind({a: 1}).call # => {}
```

Active Support defines `Proc#bind` with an analogous purpose:

```ruby
Proc.new { size }.bind([]).call # => 0
```

As you see that's callable and bound to the argument, the return value is indeed a `Method`.

NOTE: To do so `Proc#bind` actually creates a method under the hood. If you ever see a method with a weird name like `__bind_1256598120_237302` in a stack trace you know now where it comes from.

Action Pack uses this trick in `rescue_from` for example, which accepts the name of a method and also a proc as callbacks for a given rescued exception. It has to call them in either case, so a bound method is returned by `handler_for_rescue`, thus simplifying the code in the caller:

```ruby
def handler_for_rescue(exception)
  _, rescuer = Array(rescue_handlers).reverse.detect do |klass_name, handler|
    ...
  end

  case rescuer
  when Symbol
    method(rescuer)
  when Proc
    rescuer.bind(self)
  end
end
```

NOTE: Defined in `active_support/core_ext/proc.rb`.

Extensions to `Date`
--------------------

### Calculations

NOTE: All the following methods are defined in `active_support/core_ext/date/calculations.rb`.

INFO: The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is, `Date.new(1582, 10, 4).tomorrow` returns `Date.new(1582, 10, 15)` and so on. Please check `test/core_ext/date_ext_test.rb` in the Active Support test suite for expected behavior.

#### `Date.current`

Active Support defines `Date.current` to be today in the current time zone. That's like `Date.today`, except that it honors the user time zone, if defined. It also defines `Date.yesterday` and `Date.tomorrow`, and the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Date.current`.

When making Date comparisons using methods which honor the user time zone, make sure to use `Date.current` and not `Date.today`. There are cases where the user time zone might be in the future compared to the system time zone, which `Date.today` uses by default. This means `Date.today` may equal `Date.yesterday`.

#### Named dates

##### `prev_year`, `next_year`

In Ruby 1.9 `prev_year` and `next_year` return a date with the same day/month in the last or next year:

```ruby
d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
d.prev_year              # => Fri, 08 May 2009
d.next_year              # => Sun, 08 May 2011
```

If date is the 29th of February of a leap year, you obtain the 28th:

```ruby
d = Date.new(2000, 2, 29) # => Tue, 29 Feb 2000
d.prev_year               # => Sun, 28 Feb 1999
d.next_year               # => Wed, 28 Feb 2001
```

`prev_year` is aliased to `last_year`.

##### `prev_month`, `next_month`

In Ruby 1.9 `prev_month` and `next_month` return the date with the same day in the last or next month:

```ruby
d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
d.prev_month             # => Thu, 08 Apr 2010
d.next_month             # => Tue, 08 Jun 2010
```

If such a day does not exist, the last day of the corresponding month is returned:

```ruby
Date.new(2000, 5, 31).prev_month # => Sun, 30 Apr 2000
Date.new(2000, 3, 31).prev_month # => Tue, 29 Feb 2000
Date.new(2000, 5, 31).next_month # => Fri, 30 Jun 2000
Date.new(2000, 1, 31).next_month # => Tue, 29 Feb 2000
```

`prev_month` is aliased to `last_month`.

##### `prev_quarter`, `next_quarter`

Same as `prev_month` and `next_month`. It returns the date with the same day in the previous or next quarter:

```ruby
t = Time.local(2010, 5, 8) # => Sat, 08 May 2010
t.prev_quarter             # => Mon, 08 Feb 2010
t.next_quarter             # => Sun, 08 Aug 2010
```

If such a day does not exist, the last day of the corresponding month is returned:

```ruby
Time.local(2000, 7, 31).prev_quarter  # => Sun, 30 Apr 2000
Time.local(2000, 5, 31).prev_quarter  # => Tue, 29 Feb 2000
Time.local(2000, 10, 31).prev_quarter # => Mon, 30 Oct 2000
Time.local(2000, 11, 31).next_quarter # => Wed, 28 Feb 2001
```

`prev_quarter` is aliased to `last_quarter`.

##### `beginning_of_week`, `end_of_week`

The methods `beginning_of_week` and `end_of_week` return the dates for the
beginning and end of the week, respectively. Weeks are assumed to start on
Monday, but that can be changed passing an argument, setting thread local
`Date.beginning_of_week` or `config.beginning_of_week`.

```ruby
d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
d.beginning_of_week          # => Mon, 03 May 2010
d.beginning_of_week(:sunday) # => Sun, 02 May 2010
d.end_of_week                # => Sun, 09 May 2010
d.end_of_week(:sunday)       # => Sat, 08 May 2010
```

`beginning_of_week` is aliased to `at_beginning_of_week` and `end_of_week` is aliased to `at_end_of_week`.

##### `monday`, `sunday`

The methods `monday` and `sunday` return the dates for the previous Monday and
next Sunday, respectively.

```ruby
d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
d.monday                     # => Mon, 03 May 2010
d.sunday                     # => Sun, 09 May 2010

d = Date.new(2012, 9, 10)    # => Mon, 10 Sep 2012
d.monday                     # => Mon, 10 Sep 2012

d = Date.new(2012, 9, 16)    # => Sun, 16 Sep 2012
d.sunday                     # => Sun, 16 Sep 2012
```

##### `prev_week`, `next_week`

The method `next_week` receives a symbol with a day name in English (default is the thread local `Date.beginning_of_week`, or `config.beginning_of_week`, or `:monday`) and it returns the date corresponding to that day.

```ruby
d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.next_week              # => Mon, 10 May 2010
d.next_week(:saturday)   # => Sat, 15 May 2010
```

The method `prev_week` is analogous:

```ruby
d.prev_week              # => Mon, 26 Apr 2010
d.prev_week(:saturday)   # => Sat, 01 May 2010
d.prev_week(:friday)     # => Fri, 30 Apr 2010
```

`prev_week` is aliased to `last_week`.

Both `next_week` and `prev_week` work as expected when `Date.beginning_of_week` or `config.beginning_of_week` are set.

##### `beginning_of_month`, `end_of_month`

The methods `beginning_of_month` and `end_of_month` return the dates for the beginning and end of the month:

```ruby
d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_month     # => Sat, 01 May 2010
d.end_of_month           # => Mon, 31 May 2010
```

`beginning_of_month` is aliased to `at_beginning_of_month`, and `end_of_month` is aliased to `at_end_of_month`.

##### `beginning_of_quarter`, `end_of_quarter`

The methods `beginning_of_quarter` and `end_of_quarter` return the dates for the beginning and end of the quarter of the receiver's calendar year:

```ruby
d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_quarter   # => Thu, 01 Apr 2010
d.end_of_quarter         # => Wed, 30 Jun 2010
```

`beginning_of_quarter` is aliased to `at_beginning_of_quarter`, and `end_of_quarter` is aliased to `at_end_of_quarter`.

##### `beginning_of_year`, `end_of_year`

The methods `beginning_of_year` and `end_of_year` return the dates for the beginning and end of the year:

```ruby
d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
d.beginning_of_year      # => Fri, 01 Jan 2010
d.end_of_year            # => Fri, 31 Dec 2010
```

`beginning_of_year` is aliased to `at_beginning_of_year`, and `end_of_year` is aliased to `at_end_of_year`.

#### Other Date Computations

##### `years_ago`, `years_since`

The method `years_ago` receives a number of years and returns the same date those many years ago:

```ruby
date = Date.new(2010, 6, 7)
date.years_ago(10) # => Wed, 07 Jun 2000
```

`years_since` moves forward in time:

```ruby
date = Date.new(2010, 6, 7)
date.years_since(10) # => Sun, 07 Jun 2020
```

If such a day does not exist, the last day of the corresponding month is returned:

```ruby
Date.new(2012, 2, 29).years_ago(3)     # => Sat, 28 Feb 2009
Date.new(2012, 2, 29).years_since(3)   # => Sat, 28 Feb 2015
```

##### `months_ago`, `months_since`

The methods `months_ago` and `months_since` work analogously for months:

```ruby
Date.new(2010, 4, 30).months_ago(2)   # => Sun, 28 Feb 2010
Date.new(2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010
```

If such a day does not exist, the last day of the corresponding month is returned:

```ruby
Date.new(2010, 4, 30).months_ago(2)    # => Sun, 28 Feb 2010
Date.new(2009, 12, 31).months_since(2) # => Sun, 28 Feb 2010
```

##### `weeks_ago`

The method `weeks_ago` works analogously for weeks:

```ruby
Date.new(2010, 5, 24).weeks_ago(1)    # => Mon, 17 May 2010
Date.new(2010, 5, 24).weeks_ago(2)    # => Mon, 10 May 2010
```

##### `advance`

The most generic way to jump to other days is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, and returns a date advanced as much as the present keys indicate:

```ruby
date = Date.new(2010, 6, 6)
date.advance(years: 1, weeks: 2)  # => Mon, 20 Jun 2011
date.advance(months: 2, days: -2) # => Wed, 04 Aug 2010
```

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method `advance` advances first one month, and then one day, the result is:

```ruby
Date.new(2010, 2, 28).advance(months: 1, days: 1)
# => Sun, 29 Mar 2010
```

While if it did it the other way around the result would be different:

```ruby
Date.new(2010, 2, 28).advance(days: 1).advance(months: 1)
# => Thu, 01 Apr 2010
```

#### Changing Components

The method `change` allows you to get a new date which is the same as the receiver except for the given year, month, or day:

```ruby
Date.new(2010, 12, 23).change(year: 2011, month: 11)
# => Wed, 23 Nov 2011
```

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

```ruby
Date.new(2010, 1, 31).change(month: 2)
# => ArgumentError: invalid date
```

#### Durations

Durations can be added to and subtracted from dates:

```ruby
d = Date.current
# => Mon, 09 Aug 2010
d + 1.year
# => Tue, 09 Aug 2011
d - 3.hours
# => Sun, 08 Aug 2010 21:00:00 UTC +00:00
```

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

```ruby
Date.new(1582, 10, 4) + 1.day
# => Fri, 15 Oct 1582
```

#### Timestamps

INFO: The following methods return a `Time` object if possible, otherwise a `DateTime`. If set, they honor the user time zone.

##### `beginning_of_day`, `end_of_day`

The method `beginning_of_day` returns a timestamp at the beginning of the day (00:00:00):

```ruby
date = Date.new(2010, 6, 7)
date.beginning_of_day # => Mon Jun 07 00:00:00 +0200 2010
```

The method `end_of_day` returns a timestamp at the end of the day (23:59:59):

```ruby
date = Date.new(2010, 6, 7)
date.end_of_day # => Mon Jun 07 23:59:59 +0200 2010
```

`beginning_of_day` is aliased to `at_beginning_of_day`, `midnight`, `at_midnight`.

##### `beginning_of_hour`, `end_of_hour`

The method `beginning_of_hour` returns a timestamp at the beginning of the hour (hh:00:00):

```ruby
date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.beginning_of_hour # => Mon Jun 07 19:00:00 +0200 2010
```

The method `end_of_hour` returns a timestamp at the end of the hour (hh:59:59):

```ruby
date = DateTime.new(2010, 6, 7, 19, 55, 25)
date.end_of_hour # => Mon Jun 07 19:59:59 +0200 2010
```

`beginning_of_hour` is aliased to `at_beginning_of_hour`.

INFO: `beginning_of_hour` and `end_of_hour` are implemented for `Time` and `DateTime` but **not** `Date` as it does not make sense to request the beginning or end of an hour on a `Date` instance.

##### `ago`, `since`

The method `ago` receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

```ruby
date = Date.current # => Fri, 11 Jun 2010
date.ago(1)         # => Thu, 10 Jun 2010 23:59:59 EDT -04:00
```

Similarly, `since` moves forward:

```ruby
date = Date.current # => Fri, 11 Jun 2010
date.since(1)       # => Fri, 11 Jun 2010 00:00:01 EDT -04:00
```

#### Other Time Computations

### Conversions

Extensions to `DateTime`
------------------------

WARNING: `DateTime` is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example `seconds_since_midnight` might not return the real amount in such a day.

### Calculations

NOTE: All the following methods are defined in `active_support/core_ext/date_time/calculations.rb`.

The class `DateTime` is a subclass of `Date` so by loading `active_support/core_ext/date/calculations.rb` you inherit these methods and their aliases, except that they will always return datetimes:

```ruby
yesterday
tomorrow
beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
monday
sunday
weeks_ago
prev_week (last_week)
next_week
months_ago
months_since
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
prev_month (last_month)
next_month
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
years_ago
years_since
prev_year (last_year)
next_year
```

The following methods are reimplemented so you do **not** need to load `active_support/core_ext/date/calculations.rb` for these ones:

```ruby
beginning_of_day (midnight, at_midnight, at_beginning_of_day)
end_of_day
ago
since (in)
```

On the other hand, `advance` and `change` are also defined and support more options, they are documented below.

The following methods are only implemented in `active_support/core_ext/date_time/calculations.rb` as they only make sense when used with a `DateTime` instance:

```ruby
beginning_of_hour (at_beginning_of_hour)
end_of_hour
```

#### Named Datetimes

##### `DateTime.current`

Active Support defines `DateTime.current` to be like `Time.now.to_datetime`, except that it honors the user time zone, if defined. It also defines `DateTime.yesterday` and `DateTime.tomorrow`, and the instance predicates `past?`, and `future?` relative to `DateTime.current`.

#### Other Extensions

##### `seconds_since_midnight`

The method `seconds_since_midnight` returns the number of seconds since midnight:

```ruby
now = DateTime.current     # => Mon, 07 Jun 2010 20:26:36 +0000
now.seconds_since_midnight # => 73596
```

##### `utc`

The method `utc` gives you the same datetime in the receiver expressed in UTC.

```ruby
now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
now.utc                # => Mon, 07 Jun 2010 23:27:52 +0000
```

This method is also aliased as `getutc`.

##### `utc?`

The predicate `utc?` says whether the receiver has UTC as its time zone:

```ruby
now = DateTime.now # => Mon, 07 Jun 2010 19:30:47 -0400
now.utc?           # => false
now.utc.utc?       # => true
```

##### `advance`

The most generic way to jump to another datetime is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, `:hours`, `:minutes`, and `:seconds`, and returns a datetime advanced as much as the present keys indicate.

```ruby
d = DateTime.current
# => Thu, 05 Aug 2010 11:33:31 +0000
d.advance(years: 1, months: 1, days: 1, hours: 1, minutes: 1, seconds: 1)
# => Tue, 06 Sep 2011 12:34:32 +0000
```

This method first computes the destination date passing `:years`, `:months`, `:weeks`, and `:days` to `Date#advance` documented above. After that, it adjusts the time calling `since` with the number of seconds to advance. This order is relevant, a different ordering would give different datetimes in some edge-cases. The example in `Date#advance` applies, and we can extend it to show order relevance related to the time bits.

If we first move the date bits (that have also a relative order of processing, as documented before), and then the time bits we get for example the following computation:

```ruby
d = DateTime.new(2010, 2, 28, 23, 59, 59)
# => Sun, 28 Feb 2010 23:59:59 +0000
d.advance(months: 1, seconds: 1)
# => Mon, 29 Mar 2010 00:00:00 +0000
```

but if we computed them the other way around, the result would be different:

```ruby
d.advance(seconds: 1).advance(months: 1)
# => Thu, 01 Apr 2010 00:00:00 +0000
```

WARNING: Since `DateTime` is not DST-aware you can end up in a non-existing point in time with no warning or error telling you so.

#### Changing Components

The method `change` allows you to get a new datetime which is the same as the receiver except for the given options, which may include `:year`, `:month`, `:day`, `:hour`, `:min`, `:sec`, `:offset`, `:start`:

```ruby
now = DateTime.current
# => Tue, 08 Jun 2010 01:56:22 +0000
now.change(year: 2011, offset: Rational(-6, 24))
# => Wed, 08 Jun 2011 01:56:22 -0600
```

If hours are zeroed, then minutes and seconds are too (unless they have given values):

```ruby
now.change(hour: 0)
# => Tue, 08 Jun 2010 00:00:00 +0000
```

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

```ruby
now.change(min: 0)
# => Tue, 08 Jun 2010 01:00:00 +0000
```

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

```ruby
DateTime.current.change(month: 2, day: 30)
# => ArgumentError: invalid date
```

#### Durations

Durations can be added to and subtracted from datetimes:

```ruby
now = DateTime.current
# => Mon, 09 Aug 2010 23:15:17 +0000
now + 1.year
# => Tue, 09 Aug 2011 23:15:17 +0000
now - 1.week
# => Mon, 02 Aug 2010 23:15:17 +0000
```

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

```ruby
DateTime.new(1582, 10, 4, 23) + 1.hour
# => Fri, 15 Oct 1582 00:00:00 +0000
```

Extensions to `Time`
--------------------

### Calculations

NOTE: All the following methods are defined in `active_support/core_ext/time/calculations.rb`.

Active Support adds to `Time` many of the methods available for `DateTime`:

```ruby
past?
today?
future?
yesterday
tomorrow
seconds_since_midnight
change
advance
ago
since (in)
beginning_of_day (midnight, at_midnight, at_beginning_of_day)
end_of_day
beginning_of_hour (at_beginning_of_hour)
end_of_hour
beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
monday
sunday
weeks_ago
prev_week (last_week)
next_week
months_ago
months_since
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
prev_month (last_month)
next_month
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
years_ago
years_since
prev_year (last_year)
next_year
```

They are analogous. Please refer to their documentation above and take into account the following differences:

* `change` accepts an additional `:usec` option.
* `Time` understands DST, so you get correct DST calculations as in

```ruby
Time.zone_default
# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>

# In Barcelona, 2010/03/28 02:00 +0100 becomes 2010/03/28 03:00 +0200 due to DST.
t = Time.local(2010, 3, 28, 1, 59, 59)
# => Sun Mar 28 01:59:59 +0100 2010
t.advance(seconds: 1)
# => Sun Mar 28 03:00:00 +0200 2010
```

* If `since` or `ago` jump to a time that can't be expressed with `Time` a `DateTime` object is returned instead.

#### `Time.current`

Active Support defines `Time.current` to be today in the current time zone. That's like `Time.now`, except that it honors the user time zone, if defined. It also defines `Time.yesterday` and `Time.tomorrow`, and the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Time.current`.

When making Time comparisons using methods which honor the user time zone, make sure to use `Time.current` and not `Time.now`. There are cases where the user time zone might be in the future compared to the system time zone, which `Time.today` uses by default. This means `Time.now` may equal `Time.yesterday`.

#### `all_day`, `all_week`, `all_month`, `all_quarter` and `all_year`

The method `all_day` returns a range representing the whole day of the current time.

```ruby
now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now.all_day
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Mon, 09 Aug 2010 23:59:59 UTC +00:00
```

Analogously, `all_week`, `all_month`, `all_quarter` and `all_year` all serve the purpose of generating time ranges.

```ruby
now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now.all_week
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Sun, 15 Aug 2010 23:59:59 UTC +00:00
now.all_week(:sunday)
# => Sun, 16 Sep 2012 00:00:00 UTC +00:00..Sat, 22 Sep 2012 23:59:59 UTC +00:00
now.all_month
# => Sat, 01 Aug 2010 00:00:00 UTC +00:00..Tue, 31 Aug 2010 23:59:59 UTC +00:00
now.all_quarter
# => Thu, 01 Jul 2010 00:00:00 UTC +00:00..Thu, 30 Sep 2010 23:59:59 UTC +00:00
now.all_year
# => Fri, 01 Jan 2010 00:00:00 UTC +00:00..Fri, 31 Dec 2010 23:59:59 UTC +00:00
```

### Time Constructors

Active Support defines `Time.current` to be `Time.zone.now` if there's a user time zone defined, with fallback to `Time.now`:

```ruby
Time.zone_default
# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
Time.current
# => Fri, 06 Aug 2010 17:11:58 CEST +02:00
```

Analogously to `DateTime`, the predicates `past?`, and `future?` are relative to `Time.current`.

If the time to be constructed lies beyond the range supported by `Time` in the runtime platform, usecs are discarded and a `DateTime` object is returned instead.

#### Durations

Durations can be added to and subtracted from time objects:

```ruby
now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now + 1.year
#  => Tue, 09 Aug 2011 23:21:11 UTC +00:00
now - 1.week
# => Mon, 02 Aug 2010 23:21:11 UTC +00:00
```

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

```ruby
Time.utc(1582, 10, 3) + 5.days
# => Mon Oct 18 00:00:00 UTC 1582
```

Extensions to `File`
--------------------

### `atomic_write`

With the class method `File.atomic_write` you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done `atomic_write` closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like `all.css`:

```ruby
File.atomic_write(joined_asset_path) do |cache|
  cache.write(join_asset_file_contents(asset_paths))
end
```

To accomplish this `atomic_write` creates a temporary file. That's the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists `atomic_write` overwrites it and keeps owners and permissions. However there are a few cases where `atomic_write` cannot change the file ownership or permissions, this error is caught and skipped over trusting in the user/filesystem to ensure the file is accessible to the processes that need it.

NOTE. Due to the chmod operation `atomic_write` performs, if the target file has an ACL set on it this ACL will be recalculated/modified.

WARNING. Note you can't append with `atomic_write`.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

NOTE: Defined in `active_support/core_ext/file/atomic.rb`.

Extensions to `Marshal`
-----------------------

### `load`

Active Support adds constant autoloading support to `load`.

For example, the file cache store deserializes this way:

```ruby
File.open(file_name) { |f| Marshal.load(f) }
```

If the cached data refers to a constant that is unknown at that point, the autoloading mechanism is triggered and if it succeeds the deserialization is retried transparently.

WARNING. If the argument is an `IO` it needs to respond to `rewind` to be able to retry. Regular files respond to `rewind`.

NOTE: Defined in `active_support/core_ext/marshal.rb`.

Extensions to `Logger`
----------------------

### `around_[level]`

Takes two arguments, a `before_message` and `after_message` and calls the current level method on the `Logger` instance, passing in the `before_message`, then the specified message, then the `after_message`:

```ruby
logger = Logger.new("log/development.log")
logger.around_info("before", "after") { |logger| logger.info("during") }
```

### `silence`

Silences every log level lesser to the specified one for the duration of the given block. Log level orders are: debug, info, error and fatal.

```ruby
logger = Logger.new("log/development.log")
logger.silence(Logger::INFO) do
  logger.debug("In space, no one can hear you scream.")
  logger.info("Scream all you want, small mailman!")
end
```

### `datetime_format=`

Modifies the datetime format output by the formatter class associated with this logger. If the formatter class does not have a `datetime_format` method then this is ignored.

```ruby
class Logger::FormatWithTime < Logger::Formatter
  cattr_accessor(:datetime_format) { "%Y%m%d%H%m%S" }

  def self.call(severity, timestamp, progname, msg)
    "#{timestamp.strftime(datetime_format)} -- #{String === msg ? msg : msg.inspect}\n"
  end
end

logger = Logger.new("log/development.log")
logger.formatter = Logger::FormatWithTime
logger.info("<- is the current time")
```

NOTE: Defined in `active_support/core_ext/logger.rb`.

Extensions to `NameError`
-------------------------

Active Support adds `missing_name?` to `NameError`, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

TIP: A symbol can represent a fully-qualified constant name as in `:"ActiveRecord::Base"`, so the behavior for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of `PostsController` is called Rails tries optimistically to use `PostsHelper`. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that `posts_helper.rb` raises a `NameError` due to an actual unknown constant. That should be reraised. The method `missing_name?` provides a way to distinguish both cases:

```ruby
def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end
```

NOTE: Defined in `active_support/core_ext/name_error.rb`.

Extensions to `LoadError`
-------------------------

Active Support adds `is_missing?` to `LoadError`, and also assigns that class to the constant `MissingSourceFile` for backwards compatibility.

Given a path name `is_missing?` tests whether the exception was raised due to that particular file (except perhaps for the ".rb" extension).

For example, when an action of `PostsController` is called Rails tries to load `posts_helper.rb`, but that file may not exist. That's fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method `is_missing?` provides a way to distinguish both cases:

```ruby
def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "helpers/#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end
```

NOTE: Defined in `active_support/core_ext/load_error.rb`.