Removed magic comments (`# encoding: utf-8`) from tests since it's default from ruby 2 onwards

Small enhancement for generators

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

Changes a call to #map and a subsequent call to #flatten to the more idiomatic
version of #flat_map. This commit also removes an unnessecary return statement.

Fix AM#Dirty introduction example

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

Consistent foreign key name generation

A typo fix and a small clarification on `autoload_paths`

Fixes #18787.

`attributes_to_define_after_schema_loads` better describes the
difference between `attribute` and `define_attribute`, and doesn't
conflate terms since we no longer differentiate between "user provided"
and "schema provided" types.

Now that type casting has nothing to do with columns, our only usage of
this method is gone.

Fix typo in Time#change docs

Merge pull request #18612 from eileencodes/fix-validates-on-associated-record-if-parent-is-validate-false

Fix validations on child record when record parent has validate: false

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 commit 3b637802 re-introduced url helper caching but we need to
cache a separate module for Action Mailer without paths.

Cache `url_helpers`

`url_helpers` used to be memoized. This was lost in a refactoring and
this PR adds it back. We noticed this while investigating why
integration tests are slower than controller tests.

Fix Issue #15549, unbounded memory growth when saving records that have any after_create callbacks (or any associations, which makes after_create callbacks for you)

the transaction object shouldn't know so much about active record
objects, so let's push the conditionals in to the instance.

Add note about has_one :through and :dependent

Update queue_classic gem to use latest

Pre-discard flash messages

Show proper traces on Windows for the error pages

* master:
  Move required error message and changelog to Active Record
  Use public Module#include, in favor of https://bugs.ruby-lang.org/issues/8846
  Use Module#include instead of send :include, since now is a public method [ci skip]
  ✂ warning from controller renderer test

Conflicts:
	activerecord/CHANGELOG.md

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

✂ warning from controller renderer test

This is an issue brought up by @daniel-rikowski in rails/web-console#91.

Citing his PR proposal here:

> Prior to this, backtrace lines were simply split by a single colon.
>
> Unfortunately that is also the drive letter delimiter in Windows paths
> which resulted in a lot of empty source fragments of "C:0". ("C" from
> the drive letter and 0 from "/path/to/rails/file.rb:16".to_i)
>
> Now the trace line is split by the first colon followed by some digits,
> which works for both Windows and Unix path styles.

Now, the PR was sent against web-console, because of the templates copy
issue we used to had. Instead of bothering the contributor to reopen the
issue against upstream Rails itself, I will make sure he gets the credit
by putting his name in [rails-contributors/hard_coded_authors.rb][].

  [rails-contributors/hard_coded_authors.rb]: (https://github.com/fxn/rails-contributors/blob/master/app/models/names_manager/hard_coded_authors.rb).

Use public Module#include, in favor of https://bugs.ruby-lang.org/issues/8846

ref: https://github.com/rails/rails/pull/18763#issuecomment-72349769

Fix n+1 query problem when eager loading nil associations (fixes #18312)