Follow up #18914.

Its nodoc'ed for the other implementations, and doc'ed in the base
class, just like the other change_column* methods.

`change_column_null` is doc'ed only in
ActiveRecord::ConnectionAdapters::SchemaStatements, so it would make
sense to :nodoc: it elsewhere.

 - This method is used only by adapters to validate length of new index names.

 - Added documentation for index_name_exists? and rename_index.
 - Also changed rails to \Rails in documentation of
   allowed_index_name_length.

Add a test-case to make sure that `create_table` with
a `foreign_key: true` and an adapter without foreign key support
does not blow up.

Motivated by #19794.

Originating from: https://github.com/rails/rails/commit/99a6f9e60ea55924b44f894a16f8de0162cf2702#commitcomment-10855210

This reverts commit 08ff4ccb, reversing
changes made to 6c9ed6db.

Caused by #17920.

Closes #19545.

This patch introduced regressions because initializers were no longer
loaded. Specifically missing inflections result in broken restores of
the database.

Also added reference for to_date, as it belongs historically.

follow up to 107526e8

When running Active Record MySQL test, this warning is printed in the
console:

    warning: instance variable errno not initialized

It turns out that this is a warning from `mysql` gem in MySQL::Error
object. However, since the `mysql` gem is no longer maintained, and
there won't be a newer version, it make sense for us to just silence
this warning to make the output cleaner.

In 1f006c an option was added called :class to allow passing anonymous
classes to association definitions. Since using :class instead of
:class_name is a fairly common typo even amongst experienced developers
this can result in hard to debug errors arising in raise_on_type_mismatch?

To fix this we're renaming the option from :class to :anonymous_class as
that is a more correct description of what the option is for. Since this
was an internal, undocumented option there is no need for a deprecation.

Fixes #19659

This will avoid the indirection of having calling id since we already
know which is the primary key column.

Also this will make explicit the behavior since it is not clear that id
gets the right primary key value and not just the value of the "id"
column.

Fixes #19776

change test variable names and use more verbose on method

less verbose

use _read_attribute instead of send

To me it seems like this should only be the case if `autosave: true` is
set on the association. However, when implemented that way, it caused
issues with has many associations, where we have explicit tests stating
that child records are updated when the parent is new, even if autosave
is not set (presumably to update the parent id, but other changed
attributes would be persisted as well).

It's quirky, but at least we should be consistently quirky. This
constitutes a minor but subtle change in behavior, and therefore should
not be backported to 4.2 and earlier.

Fixes #19782

IMO we shouldn't encourage users to use methods they shouldn't need to know about. As Song (in this example) inherits from ActiveRecord, we can use super here instead to get the same effect with the bonus of not knowing how active record actually implements these methods.

This is a squash of the following commits, from first to last:

-

Fix minor, random things I’ve come across lately that individually
did not seem worth making a PR for, so I saved them for one commit.

One common error is using “it’s” (which is an abbreviation of “it is”)
when the possessive “its” should be used for indicating possession.

-

Changes include the name of a test, so remove the `[skip ci]` (thanks @senny).

-

Line wrap the changes at 80 chars and add one more doc fix.

-

Add a missing line wrap in the Contributing to Ruby on Rails Guide.

-

Line wrap the `TIP` section in the Contributing to Ruby on Rails Guide as well.

Rendering the guide locally with `bundle exec rake guides:generate` did
not show any change in on-screen formatting after adding the line wrap.

The HTML generated is (extra line added to illustrate where the line
wrap takes place):

    <div class="info"><p>Please squash your commits into a single commit
when appropriate. This

    simplifies future cherry picks and also keeps the git log
clean.</p></div>

-

Squash commits.

The `index` option used with `timestamps` should be passed to both
`column` definitions for `created_at` and `updated_at` rather than just
the first.

This was happening because `Hash#delete` is used to extract the `index`
option passed to `timestamps`, thereby mutating the `options` hash
in-place. Now take a copy of the `options` before deleting so that the
original is not modified.

This reverts commit 524d4059, reversing
changes made to 34d3a609.

Reasoning behind the revert are in the PR discussion:
https://github.com/rails/rails/pull/19755

- This means that types can no longer cast to/from `Set`, and reasonably
  work with `where` (we already have this problem for `array`/`json`
  types on pg)
- This adds precedent for every other `Enumerable`, and we can't target
  `Enumerable` directly.
- Calling `to_a` on a `Set` is reasonable.

Previously `#where` used to treat `Set`objects as nil, but now it treats
them as an array:

  set = Set.new([1, 2])
  Author.where(:id => set)
  # => SELECT "authors".* FROM "authors" WHERE "authors"."id" IN (1, 2)

and fix the grammar in the ruby_version_check.rb user message.

Fixes #18106

We should document current behavior, and this is design of API for now.

Closes #17597

Documentation is giving an example that can be replaced by a more dry command.
Give a hint that ids can be used instead of pluck(:id).

We don't actually need to enumerate the possible types here any more;
that dates back to before e105e599, when
they were symbols, and indistinguishable from other options.

Without this note, someone can misunderstand the usage of validates_presence_of method

add missing note for the validates_presence_of

It fixes the strange error saying undefined method `take'.

```
RelationTest#test_find_by_without_arg_behaves_same_with_find_by({}):
NoMethodError: undefined method `take' for #<ActiveRecord::QueryMethods::WhereChain:0x007f9c55db1d68>
```

`use_transactional_fixtures` was deprecated in favor of
`use_transactional_tests` in Rails 5.0. This removes one warning while
running test suite.

The implementation of the generation of the foreign key name was changed
between Rails 4.2.0 and 4.2.1 from a random to a deterministic behavior,
however the documentation still describes the old randomized behavior.