Removed non-standard and unused require 'active_support/deprecation' from parts out of active_support.

The name `ActiveModel::AttributeAssignment::UnknownAttributeError` is
too implementation specific so let's move the constant directly under
the ActiveModel namespace.

Also since this constant used to be under the ActiveRecord namespace, to
make the upgrade path easier, let's avoid raising the former constant
when we deal with this error on the Active Record side.

Previously, calling `User.model_name.to_json` would result in an infinite
recursion as `.model_name` inherited its `.as_json` behavior from Object. This
patch fixes that unexpected behavior by delegating `.as_json` to :name.

ActiveModel::Errors

without replacement.

The existing example seems somewhat forced: is it realistic
to have a model that accepts state in its initializer but
considers it has not been changed? By allowing state changes
to happen only via accessors it seems more natural that new
instances are considered to be unchanged (as they are in AR).

[ci skip]

As per Rails general coding conventions. Related to #18794 [ci skip]

Added simple initialize and made use of Person.new instead of Person.find_by to clarify the docs.
[ci skip]

Fixes #17621. This 5 year old (or older) issue causes validations to fire
when a parent record has `validate: false` option and a child record is
saved. It's not the responsibility of the model to validate an
associated object unless the object was created or modified by the
parent.

Clean up tests related to validations

`assert_nothing_raised` is not benefiting us in these tests
Corrected spelling of "respects"
It's better to use `assert_not_operator` over `assert !r.valid`

The new association error belongs to Active Record, not Active Model.
See #18700 for reference.

They have inconsistent behaviour currently.

- Grammar fixes
- Add doc for changes_include?
-  implemntations => implementations

Fixes #18696.

Minor style changes across the board. Changed an alias to an explicit
method declaration, since the alias will not be documented otherwise.

Allows to use it for any object as an includable module.

To be able to return type of validator, one can now call `details`
on Errors instance:

```ruby
class User < ActiveRecord::Base
  validates :name, presence: true
end
```

```ruby
user = User.new; user.valid?; user.errors.details
=> {name: [{error: :blank}]}
```

When an attribute is assigned, we determine if it was already marked as
changed so we can determine if we need to clear the changes, or mark it
as changed. Since this only affects the `attributes_changed_by_setter`
hash, in-place changes are irrelevant to this process. Since calculating
in-place changes can be expensive, we can just skip it here.

I also added a test for the only edge case I could think of that would
be affected by this change.

Remove attributes_protected_by_default reference, since MassAssignmentSecurity was removed from ActiveModel f8c9a4d3

This commit changes the original documentation of ActiveModel::Lint::Tests
introduced in dbf20c2d to focus less on *why* the tests exist and more on
*what* the tests do.

For instance, `test_to_key` was documented as:

> Returns an Enumerable of all (primary) key attributes...

whereas `test_to_key` is simply a test meant to *fail* or *pass*, and the
documentation above refers to `to_key`.

[ci skip]

'_singularize' only ever gets called with one argument

These requires were added only to change deprecation message

Before this commit, returning `false` in an ActiveModel `before_` callback
such as `before_create` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

Before this commit, returning `false` in an ActiveModel validation
callback such as `before_validation` would halt the callback chain.

After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.

The preferred way to halt a callback chain is to explicitly `throw(:abort)`.

This commit changes arguments and default value of CallbackChain's :terminator
option.

After this commit, Chains of callbacks defined **without** an explicit
`:terminator` option will be halted as soon as a `before_` callback throws
`:abort`.

Chains of callbacks defined **with** a `:terminator` option will maintain their
existing behavior of halting as soon as a `before_` callback matches the
terminator's expectation. For instance, ActiveModel's callbacks will still
halt the chain when a `before_` callback returns `false`.

Also prevents the word "Model" from linking to the documentation
of ActiveModel::Model because that's not intended.

[ci skip]

Calling `changed_attributes` will ultimately check if every mutable
attribute has changed in place. Since this gets called whenever an
attribute is assigned, it's extremely slow. Instead, we can avoid this
calculation until we actually need it.

Fixes #18029

The default value for the argument `message` in
`ActiveModel::Errors#add` has a new behavior
since ca99ab24.

Before
  person.errors.add(:name, nil)
  # => ["is invalid"]

After
  person.errors.add(:name, nil)
  # => [nil]