This partly reverts #35617.

#35617 includes unused code (for `InsertStatement`) and re-using query
annotation for `update_all` and `delete_all`, which has not been
discussed yet.

If a relation has any annotation, I think it is mostly for SELECT query,
so re-using annotation by default is not always desired behavior for me.

We should discuss about desired behavior before publishing the
implementation.

When adding a child record via a has_many :through association,
build_through_record would previously build the join record, and then
assign the child record and source_type option to it.  Because the
before_add and after_add callbacks are called as part of build, however,
this caused the callbacks to receive incomplete records, specifically
without the other end of the has_many :through association.  Collecting
all attributes before building the join record ensures the callbacks
receive the fully constructed record.

* Add `ActiveRecord::Relation#extract_associated` for extracting associated records from a relation

Before 34cc301f, type casting by boolean attribute when querying is a
no-op, so finding by truthy boolean string (i.e.
`where(value: "true") # => value = 'true'`) didn't work as expected
(matches it to FALSE in MySQL #32624). By type casting is ensured, a
value on boolean attribute is always serialized to TRUE or FALSE.

In PostgreSQL, `where(value: :false) # => value = 'false'` was a valid
SQL, so 34cc301f is a regresson for PostgreSQL since all symbol values
are serialized as TRUE.

I'd say using `:false` is mostly a developer's mistake (user's input
basically comes as a string), but `:false` on boolean attribute is
serialized as TRUE is not a desirable behavior for anybody.

This allows falsy boolean symbols as false, i.e.
`klass.create(value: :false).value? # => false` and
`where(value: :false) # => value = FALSE`.

Fixes #35676.

We have a test case for `collation_connection` session variable, so it
should not be changed in any other test.

Fixes #35458.

`TRUNCATE TABLE posts` also resets `AUTO_INCREMENT`. If newly created a
post, it is wrongly associated with remaining tagging records.
To un-associate remaining tagging record, use `post.create_tagging!`
instead.

Fixes #35751.

Fix #35665

```ruby
$ ARCONN=mysql2 bin/test test/cases/scoping/named_scoping_test.rb test/cases/tasks/database_tasks_test.rb test/cases/associations/cascaded_eager_loading_test.rb test/cases/associations/eager_singularization_test.rb -n "/^(?:NamedScopingTest#(?:test_many_should_not_fire_query_if_scope_loaded)|ActiveRecord::DatabaseTasksDumpSchemaCacheTest#(?:test_dump_schema_cache)|CascadedEagerLoadingTest#(?:test_eager_association_loading_with_has_many_sti_and_subclasses)|EagerSingularizationTest#(?:test_eager_no_extra_singularization_has_many_through_belongs_to))$/" --seed 16818
Using mysql2
Run options: -n "/^(?:NamedScopingTest#(?:test_many_should_not_fire_query_if_scope_loaded)|ActiveRecord::DatabaseTasksDumpSchemaCacheTest#(?:test_dump_schema_cache)|CascadedEagerLoadingTest#(?:test_eager_association_loading_with_has_many_sti_and_subclasses)|EagerSingularizationTest#(?:test_eager_no_extra_singularization_has_many_through_belongs_to))$/" --seed 16818

...F

Failure:
CascadedEagerLoadingTest#test_eager_association_loading_with_has_many_sti_and_subclasses [/home/yahonda/git/rails/activerecord/test/cases/associations/cascaded_eager_loading_test.rb:124]:
1 instead of 0 queries were executed.
Queries:
SHOW FULL FIELDS FROM `topics`.
Expected: 0
  Actual: 1

bin/test test/cases/associations/cascaded_eager_loading_test.rb:119

Finished in 6.894609s, 0.5802 runs/s, 1.0153 assertions/s.
4 runs, 7 assertions, 1 failures, 0 errors, 0 skips
$
```

Fixes #35680

The problem occurred, when a `has one through` association contains
a foreign key (it belongs to the intermediate association).

For example, Comment belongs to Post, Post belongs to Author, and Author
`has_one :first_comment, through: :first_post`.

In this case, the value of the foreign key is comparing with the original
record, and since they are likely different, the association is marked
as changed. So it updates every time when the origin record updates.

This patch has two main portions:

1. Add SQL comment support to Arel via Arel::Nodes::Comment.
2. Implement a Relation#annotate method on top of that.

== Adding SQL comment support

Adds a new Arel::Nodes::Comment node that represents an optional SQL
comment and teachers the relevant visitors how to handle it.

Comment nodes may be added to the basic CRUD statement nodes and set
through any of the four (Select|Insert|Update|Delete)Manager objects.

For example:

    manager = Arel::UpdateManager.new
    manager.table table
    manager.comment("annotation")
    manager.to_sql # UPDATE "users" /* annotation */

This new node type will be used by ActiveRecord::Relation to enable
query annotation via SQL comments.

== Implementing the Relation#annotate method

Implements `ActiveRecord::Relation#annotate`, which accepts a comment
string that will be appeneded to any queries generated by the relation.

Some examples:

    relation = Post.where(id: 123).annotate("metadata string")
    relation.first
    # SELECT "posts".* FROM "posts" WHERE "posts"."id" = 123
    # LIMIT 1 /* metadata string */

    class Tag < ActiveRecord::Base
      scope :foo_annotated, -> { annotate("foo") }
    end
    Tag.foo_annotated.annotate("bar").first
    # SELECT "tags".* FROM "tags" LIMIT 1 /* foo */ /* bar */

Also wires up the plumbing so this works with `#update_all` and
`#delete_all` as well.

This feature is useful for instrumentation and general analysis of
queries generated at runtime.

This has been supported for a while but we didn't have documentation
for it.

I found `:unique_by` with `:columns` and `:where` inside it tough to
grasp. The documentation only mentioned indexes and partial indexes.
So why duplicate a model's indexes in an insert_all/upsert_all call
when we can just look it up?

This has the added benefit of raising if no index is found, such that
people can't insert thousands of records without relying on an index
of some form.

Internal usage for the method as public has removed at #29623.

That is completely covered by `Arel::Nodes::ValuesList`.

Related https://github.com/rails/arel/pull/484.

Before:

```
   (16.4ms)  TRUNCATE TABLE `author_addresses`
   (20.5ms)  TRUNCATE TABLE `authors`
   (19.4ms)  TRUNCATE TABLE `posts`
```

After:

```
  Truncate Tables (19.5ms)  TRUNCATE TABLE `author_addresses`;
TRUNCATE TABLE `authors`;
TRUNCATE TABLE `posts`
```

This is to easier make `truncate_tables` to bulk statements.

We as Arm Treasure Data are using Optimizer Hints with a monkey patch
(https://gist.github.com/kamipo/4c8539f0ce4acf85075cf5a6b0d9712e),
especially in order to use `MAX_EXECUTION_TIME` (refer #31129).

Example:

```ruby
class Job < ApplicationRecord
  default_scope { optimizer_hints("MAX_EXECUTION_TIME(50000) NO_INDEX_MERGE(jobs)") }
end
```

Optimizer Hints is supported not only for MySQL but also for most
databases (PostgreSQL on RDS, Oracle, SQL Server, etc), it is really
helpful to turn heavy queries for large scale applications.

This reverts commit 65f2eeaa.

Currently GeneratedAttributeMethods is a module builder class, an
instance of which is included in every AR class. OTOH,
GeneratedAssociatedMethods is assigned to a constant under the model
namespace. This is inconsistent and looks strange in the list of
ancestors.

There is no particular reason *not* to assign a constant for this (very
important) module under the model namespace, so that's what this commit
does.

Previous to this change, ancestors for an AR class looked like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace0f05b08>,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace093c460>,
 ActiveRecord::Base,
 ...
```

With this change, they look like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 User::GeneratedAttributeMethods,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 ApplicationRecord::GeneratedAttributeMethods,
 ActiveRecord::Base,
 ...
```

The previously named `GeneratedAttributeMethods` module builder class is
renamed `GeneratedAttributeMethodsBuilder` to emphasize that this is not
a module but a class.

Useful to not query for indexes when an application uses schema cache.

Ref https://github.com/rails/rails/pull/35546

YAML has been used to serialize the schema cache ever since 2016 with
Rails 5.1: 4c00c6ed

This test is only executed for OracleAdapter.

https://github.com/rsim/oracle-enhanced/pull/1846 adds
an equivalent spec for Oracle enhanced adapter.

An `author` has a lots of `posts` in the fixtures, so the result of
`author.post` and finding a `post` by `author_id` is non-deterministic.

https://travis-ci.org/rails/rails/jobs/504332292#L1202-L1208

change the line to check an attribute has user-defined type

ref: https://github.com/rails/rails/pull/35320#discussion_r257924552

check query attribute method is working when given value does not respond to to_i method

`disconnect!` will lose all tables and fixtures if in memory db.

This patch fixes the issue when association is preloaded with a custom
preload scope which disposes the already preloaded target of the
association by reseting it.

When custom preload scope is used, the preloading is now performed into
a separated Hash - #records_by_owner instead of the association.
It removes the necessaty the reset the association after the preloading
is complete so that reset of the preloaded association never happens.

Preloading is still happening to the association when the preload scope
is empty.

I've found the skewness of delegation methods between `except` and
`only` in a88b6f25.

The `only` method is closely similar with `except` as `SpawnMethods`.

https://github.com/rails/rails/blob/e056b9bfb07c4eb3bcc6672d885aadd72bec574f/activerecord/lib/active_record/relation/spawn_methods.rb#L53-L67

It is preferable both behaves the same way.

This makes to ease testing `QUERYING_METHODS`.