Revert "Remove useless `column_alias` in `subquery_for_count`"

This reverts commit 28977f1f.

`primary_key` and `references` columns should be identical type

Move some ActionCable logs to debug level

Decouple the building Arel ASTs for uniqueness validator

If select clause is specified and last column has a column alias,
additional column alias causes a statement invalid.

Add test coverage for counting a single column with NULL values.

Fixes #27676, #27682, and #27705.

Follow up to #26266.

The default type of `primary_key` and `references` were changed to
`bigint` since #26266. But legacy migration and sqlite3 adapter should
keep its previous behavior.

Restore the behaviour of the compatibility layer for integer-like PKs

* kamipo/fix_mysql_pk_dumping_correctly:
  Restore custom primary key tests lost at #26266
  Restore the behaviour of the compatibility layer for integer-like PKs
  Correctly dump integer-like primary key with default nil

Deprecate `ColumnDumper#migration_keys`

Remove superfluous `MySQL::Column` tests

`ColumnDumper#migration_keys` was extracted to customize keys for
standardized column arguments widths. But the feature was removed in
df84e986. The internal method is no longer used for that.

Since text default treated as an empty string in non-strict mode has
been removed in #26154, `MySQL::Column` behaves like any other column.
Also, The difference between strict and non-strict mode is covered by
`test_mysql_not_null_defaults_strict` and `test_mysql_not_null_defaults_non_strict`.

Remove `ActiveModel::TestCase` from lib

`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.

Correct spelling

Add test for DateTime#blank?

Indicate action that failed in YamlColumn

```
go get -u github.com/client9/misspell/cmd/misspell
misspell  -w -error -source=text .
```

Add `:args` to `process.action_mailer` event.

Collect all file patterns when running multiple rake test tasks

Docs: Correction: Module::DelegationError

Replaces the rake_patterns instance variable with simple require, as
`autorun` will run tests from all eagerly required test files.

Fixes #27801

Some custom primary key tests (added at #17631, #17696, #18220, #18228)
were lost at #26266. Restore the tests to improve test coverage.

The PR #27384 changed migration compatibility behaviour.

```ruby
class CreateMasterData < ActiveRecord::Migration[5.0]
  def change
    create_table :master_data, id: :integer do |t|
      t.string :name
    end
  end
end

```

Previously this migration created non-autoincremental primary key
expected. But after the PR, the primary key changed to autoincremental,
it is unexpected.

This change restores the behaviour of the compatibility layer.

The PR #27384 changed integer-like primary key to be autoincrement
unless an explicit default. This means that integer-like primary key is
restored as autoincrement unless dumping the default nil explicitly.
We should dump integer-like primary key with default nil correctly.

When the delegation target is nil and the allow_nil option is not
in use, a Module::DelegationError is raised.

    class C
      delegate :a, to: :b
        def b
          nil
        end
      end
    C.new.a
    # => Module::DelegationError: C#a delegated to b.a, but b is nil

[ci skip]

Fix typo in guide: _articles should be _article

The guide contains a typo in the "local_assigns" section, where
rendering a model named 'Article' via `render @articles` is shown to
render a partial called `_articles.html.erb`, when in fact the necessary
partial name is `_article.html.erb`

Fix collection_singular_ids= bug

Fix a test of AR::Type::TypeMap#lookup when using Oracle

Ignore yarn error log file

When the association's primary key is manually set as a symbol and called with an array of strings, CollectionAssociation#ids_writer
fails to cast the ids to integers. This is because
AssociationReflection#association_primary_key_type and
ThroughReflection#association_primary_key_type return the incorrect
type, since ModelSchema.type_for_attribute only accepts a string. The
result is an ActiveRecord::RecordNotFound error.

generate mailer layout files only when it does not exist

Fix CI failure caused by aa647b46