[ci skip]

Fixes #29512.

The output of two string attributes is displayed differently in the docs. Standardize the output by always showing it as a comment.

* Allow a default value to be declared for class_attribute

* Convert to using class_attribute default rather than explicit setter

* Removed instance_accessor option by mistake

* False is a valid default value

* Documentation

a user input for a decimal column were ignored by numerically validations

[ci skip]

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

When define callbacks latest definition on the same callback/method
overwrites previous ones.

* Remove trailing spaces.
* Add backticks around method and command.
* Fix indentation.

\Z was a mistake of \z. Replace \Z to \z to prevent newly \Z added.

Follow up of #17148.

We already have a _read_attribute method that can get the value we need
from the model.  Lets define that method in AM::Dirty and use the
existing one from AR::Dirty rather than introducing a new method.

[ci skip]

See 34321e4a for background on
ImmutableString vs String.

Our String type cannot delegate typecasting to ImmutableString, because
the latter freezes its input: duplicating the value after that gives us
an unfrozen result, but still mutates the originally passed object.

```ruby
nil.respond_to?(:stringify_keys) # => false
```

```ruby
include ActiveModel::AttributeAssignment
Object.new.assign_attributes(nil) #  => ArgumentError: When assigning attributes, you must pass a hash as an argument.
```

`messages` has `default_proc` so calling `reject` causes the warning.

https://github.com/ruby/ruby/blob/v2_4_1/hash.c#L1335-L1337

This CHANGELOG.md is a continuation of the 5-1-stable one, there
shouldn't be any duplicate entries.

[ci skip]

Before:
  person.errors.keys    # => []
  person.errors.values  # => []
  person.errors[:name]  # => []
  person.errors.keys    # => [:name]
  person.errors.values  # => [[]]

After:
  person.errors.keys   # => []
  person.errors.values # => []
  person.errors[:name] # => []
  person.errors.keys   # => []
  person.errors.values # => []

Related to #23468

In Ruby 2.4, BigDecimal(), as used by the Decimal cast, was changed so
that it will raise ArgumentError when passed an invalid string, in order
to be more consistent with Integer(), Float(), etc. The other numeric
types use ex. to_i and to_f.

Unfortunately, we can't simply change BigDecimal() to to_d. String#to_d
raises errors like BigDecimal(), unlike all the other to_* methods (this
should probably be filed as a ruby bug).

Instead, this simulates the existing behaviour and the behaviour of the
other to_* methods by finding a numeric string at the start of the
passed in value, and parsing that using BigDecimal().

See also
https://bugs.ruby-lang.org/issues/10286
https://github.com/ruby/bigdecimal/commit/3081a627cebdc1fc119425c7a9f009dbb6bec8e8

This issue is only appear when you try to call `define_attribute_method`
and passing a symbol in Active Record. It does not appear in isolation
in Active Model itself.

Before this patch, when you run `User.define_attribute_method :foo`, you
will get:

    NoMethodError: undefined method `unpack' for :foo:Symbol
        from activerecord/lib/active_record/attribute_methods/read.rb:28:in `define_method_attribute'
        from activerecord/lib/active_record/attribute_methods/primary_key.rb:61:in `define_method_attribute'
        from activemodel/lib/active_model/attribute_methods.rb:292:in `block in define_attribute_method'
        from activemodel/lib/active_model/attribute_methods.rb:285:in `each'
        from activemodel/lib/active_model/attribute_methods.rb:285:in `define_attribute_method'

This patch contains both a fix in Active Model and a test in Active
Record for this error.

The `:doc:` was added in cdb9d7f4 but `NumericalityValidator` is already
`:nodoc:` class. `:doc:` is unneeded.

https://github.com/rails/rails/blob/master/activemodel/lib/active_model/validations/numericality.rb#L3

These files are not using `strip_heredoc`.

Closes #27976

`ActiveModel::TestCase` is used only for the test of Active Model.
Also, it is a private API and can not be used in applications.
Therefore, it is not necessary to include it in lib.