With this we can write `Model#select(:aliased)`, `Model#order(:aliased)`,
`Model#reoder(aliased: :desc)`, etc.

Supplementary work to 54122067.

Add docs about behaviour of replacing a has_one associate object.

Add docs about behaviour of replacing a has_one associate object, that the previous one is deleted even if the new one
 doesn't get persisted to database.

 Fixes #13197 . [ci skip]

Replace File.exists? with File.exist?

To quell warnings on ruby 2.1

use the new clear_validators! api everywhere to reset validators in tests

Rails config for raise on missing translations

The request attributes filtered_parameters, filtered_env and filtered_path
are memoized for performance reasons. However this can cause unusual
behavior in tests where there are multiple calls to get, post, etc.

Fixes #13803.

Ability to specify multiple contexts when defining a validation.

Add a config to setup whether raise exception for missing translation or
not.

Display value when raising due to unscope() issues

Example:

validates_presence_of :name, on: [:update, :custom_validation_context]

Ruby on Rails 4.1 Release Notes: Fix spelling [ci skip]

Hopefully make it easier to debug errors. e.g

Before:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String is unimplemented.

After:

    RuntimeError:
       unscope(where: "deleted_at") failed: unscoping String "'t'='t'" is unimplemented.

Both Time#xmlschema and DateTime#xmlschema can accept nil values for the
fraction_digits parameter. This commit makes this so for TimeWithZone
values as well.

The subsecond fraction digits had been hardcoded to 3. This forced all
timestamps to include the subsecond digits with no way to customize the
value. While the subsecond format is part of the ISO8601 spec, it is not
adhered to by all parsers (notably mobile clients). This adds the
ability to customize the number of digits used, optionally setting them
to 0 in order to eliminate the subsecond fraction entirely:

ActiveSupport::JSON::Encoding.subsecond_fraction_digits = 0

Modern RDoc accepts a few more things in +...+. In particular symbols work now.

The current regexp in RDoc is https://github.com/rdoc/rdoc/blob/v4.1.1/lib/rdoc/markup/attribute_manager.rb#L133.

Closes #11247.

Fix order syntax in find_by_sql example [ci skip]

Currently if a time is changed during DST overlap in the autumn then the
method `period_for_local` will return the DST period. However if the
original time is not DST then this can be surprising and is not what is
generally wanted. This commit changes that behavior to maintain the current
period if it's in the list of periods returned by `periods_for_local`.

It is possible to alter the behavior of `period_for_local` by specifying a
second argument but since we may be change from another time that could be
either DST or not then this would give inconsistent results.

Fixes #12163.

We allow the use of underscored symbols to represent classes throughout
other parts of Rails so it seems incongruous that it's not supported in
`register_interceptor` and `register_observer`.

Since preview_path is read from ActionMailer::Base when previewing, subclasses
can’t change it so don’t there's no need for the extra overhead imposed by using it.

To support the ability for tools like CSS style inliners to operate on emails
being previewed this commit adds a hook in a similar fashion to the existing
delivery interceptor hook, e.g:

  class CSSInlineStyler
    def self.previewing_email(message)
      # inline CSS styles
    end
  end

  ActionMailer::Base.register_preview_interceptor CSSInlineStyler

Fixes #13622.

thread -> threaded [ci skip]