This option is unused, left over from pre-1.0 Rails to internally distinguish the location of the foreign key.

In addition to defining the association, a `required` association will
also have its presence validated.

Before:

```ruby
belongs_to :account
validates_presence_of :account
```

After:

```ruby
belongs_to :account, required: true
```

This helps to draw a distinction between types of validations, since
validations on associations are generally for data integrity purposes,
and aren't usually set through form inputs.

Now the following case will work fine

    class Tag < ActiveRecord::Base
    end

    class Publisher::Article < ActiveRecord::Base
      has_and_belongs_to_many :tags
    end

Fixes #15761

Fix habtm reflection
Conflicts:
    activerecord/CHANGELOG.md
    activerecord/lib/active_record/counter_cache.rb
    activerecord/lib/active_record/reflection.rb
    activerecord/test/cases/reflection_test.rb

Fixes #14824.

cc:
https://github.com/rails/rails/commit/a11e2fcff8cdc9691b977d8330abe63024f8c732#commitcomment-6353614
and 0369dbfd

parent_name is a string of nil, never a blank string so we don't need to
use presence.

Thank's for @laurocaetano for the help with tests. 😃

Fixes #14709

When used a custom join_table name on a habtm, rails was not saving it
on Reflections. This causes a problem when rails loads fixtures, because
it uses the reflections to set database with fixtures.

Dangerous association names conflicts include instance or class
methods already defined by `ActiveRecord::Base`.

Fixes: #13445

activerecord-deprecated_finders expects the parameters denormalized in
its initialize method

This is to get activerecord-deprecated_finders work again

It is need in activerecord-deprecated_finders

Example: Given you have a comments model with a polymorphic commentable
association (e.g. books and songs) with the touch option set.
Every time you update a comment its commentable should be touched.
This was working when you changed attributes on the comment or when you
moved the comment from one book to another. However, it was not working
when moving a comment from a book to a song. This is now fixed.

Previously, the `has_one` macro incorrectly accepts the `counter_cache` option
due to a bug, although that options was never supported nor functional on
`has_one` and `has_one ... through` relationships. It now correctly raises an
`ArgumentError` when passed that option.

For reference, this bug was introduced in 52f8e4b9.

All the job can be done at class level so we can avoid some object
allocation

instance

the instance