`test_middleware_caches` is sometimes failed since #29454.
The failure is due to schema statements are affected by query caching.
Bypassing query caching for schema statements to avoid the issue.

Fixes #29627.

Active Record doesn't rely delegating to `arel` in the internal since
425f2cac. The delegation is a lower priority than delegating to `klass`,
so it is pretty unclear which method is delegated to `arel`.

For example, `bind_values` method was removed at b06f64c3 (a series of
changes https://github.com/rails/rails/compare/79f71d3...b06f64c). But a
relation still could respond to the method because `arel` also have the
same named method (#28976).

Removing the delegation will achieve predictable behavior.

`test_cache_is_available_when_using_a_not_connected_connection` is
always failed if running only the test since #29609.

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/query_cache_test.rb -n test_cache_is_available_when_using_a_not_connected_connection
Using mysql2
Run options: -n test_cache_is_available_when_using_a_not_connected_connection --seed 15043

F

Finished in 0.070519s, 14.1806 runs/s, 28.3612 assertions/s.

  1) Failure:
QueryCacheTest#test_cache_is_available_when_using_a_not_connected_connection [test/cases/query_cache_test.rb:336]:
2 instead of 1 queries were executed.
Queries:
SELECT  `tasks`.* FROM `tasks` WHERE `tasks`.`id` = ? LIMIT ?
SET NAMES utf8 COLLATE utf8_unicode_ci,  @@SESSION.sql_mode = CONCAT(CONCAT(@@sql_mode, ',STRICT_ALL_TABLES'), ',NO_AUTO_VALUE_ON_ZERO'),  @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483.
Expected: 1
  Actual: 2

1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```

This failure is due to `LogSubscriber` will use not connected
`ActiveRecord::Base.connection` even if `Task.connection` is connected.
I fixed to always pass `type_casted_binds` to log subscriber to avoid
the issue.

This regression was caused by #23004.

If STI subclass is using scoping in parent class scoping,
`current_scope` in subclass is never restored.
I fixed to restore `current_scope` to previous value correctly.

`aliased_table_name` in `Association` was added at a3502c41.
`aliased_table_name` in `JoinDependency` (added at 55854c41) is used, but
it looks like that added one in `Association` is never used from the
beginning.

Because identifiers in SQL could include a single quote.

Related #24950, #26784.

The delegation was needed since passing `relation` with
`relation.bound_attributes`. It should use `relation.arel` in that case.

`test_middleware_caches` also failed same as #29600.

https://travis-ci.org/rails/rails/jobs/248017174#L487-L489

ActiveRecord query cache is available when a connection is connected.
Therefore, query cache is unavailable when entering the ActiveRecord::Base.cache block without being connected.

```ruby
ActiveRecord::Base.cache do
  Task.find(1) # access to database.
  Task.find(1) # access to database. unavailable query cache
end
```

If we use query cache with batch script etc, we need to connect before that.
```ruby
Task.connection

ActiveRecord::Base.cache do
  Task.find(1) # access to database.
  Task.find(1) # available query cache
end
```

Before version 3.1, query cache had been enabled if a configuration was set up.
In order to solve the `DATABASE_URL` issue(#8074), ActiveRecord has checked whether a connection is connected or not.

Today, ActiveRecord.configurations respect `DATABASE_URL`.
https://github.com/rails/rails/blob/master/activerecord/lib/active_record/core.rb#L46

Passing `klass.connection` is redundant because `AssociationScope` is
passed an association itself and an association has `klass`.

Unscoping `default_scope` in associations has already supported (#17360
for preloading, c9cf8b8d for eager loading).

Fixes #20679.
Closes #16531.

```ruby
  car = Car.create!(name: "Tofaş")

  # Before
  car.bulb_ids # => SELECT "bulbs".ID FROM "bulbs" WHERE "bulbs"."name" = $1 AND "bulbs"."car_id" = $2  [["name", "defaulty"], ["car_id", 3]]

  # After
  car.bulb_ids # => SELECT "bulbs"."ID" FROM "bulbs" WHERE "bulbs"."name" = $1 AND "bulbs"."car_id" = $2  [["name", "defaulty"], ["car_id", 3]]
```

And use `assert_same` instead of `assert_equal` and tiny fix assert
message s/#reload/#reset/.

Follow up of #29511.

If a record was built on a HasManyThroughAssociation, then removed, and
then the record was saved, the removed record would be created anyways.

`test_exceptional_middleware_clears_and_disables_cache_on_error` in
postgresql adapter sometime fails recently. Show the query cache keys to
investigte the cause.

https://travis-ci.org/rails/rails/jobs/246467252#L490-L493

Simply we can skip instantiating `NullPreloader` if `assoc.klass` is
nil.

Covering #27994 in tests.

Closes #27994.

The AliasTracker#aliased_table_for needs the type caster for the joined association, not the join root

This fixes the following warnings:

```
activerecord/test/cases/relation_test.rb:231: warning: assigned but unused variable - authors_with_commented_posts
```

- On Mysql, some command line options that affect option-file handling such as `--no-defaults` have to be passed before any other options
- Modified rails to pass them right after the `mysql` command
- Ref https://dev.mysql.com/doc/refman/5.7/en/option-file-options.html and https://bugs.mysql.com/bug.php?id=83386
- Ref #27437

Scalar values like arrays and hashes can't be inserted
directly into table. Previously, the way to determine if
the value is scalar was to try quoting it. If `quote` raised
with an error than the value has to be converted to YAML.

This flow is not very obvious. Ideally we could have
a `quotable?` method in the connection, but I think
that we can avoid begin/rescue block by simply checking
if the value is Array or Hash.

https://github.com/rails/rails/commit/aa31d21f5f4fc4d679e74a60f9df9706da7de373

Doing `Author.joins(:posts).merge(Post.joins(:comments))` does this
`SELECT ... INNER JOIN posts ON... LEFT OUTER JOIN comments ON...`
instead of doing
`SELECT ... INNER JOIN posts ON... INNER JOIN comments ON...`.

This behavior is unexpected and makes little sense as, basically, doing
`Post.joins(:comments)` means I want posts that have comments. Turning
it to a LEFT JOIN means I want posts and join the comments data, if
any.

We can see this problem directly in the existing tests.
The test_relation_merging_with_merged_joins_as_symbols only does joins
from posts to comments to ratings while the ratings fixture isn't
loaded, but the count is non-zero.

The `@offsets` cache is used by FinderMethods to cache records found by
find_nth. This cache is cleared in AR::Relation#reset, but not in
CollectionProxy#reset or CollectionProxy#reload.

Because of this, since #29098, calling #first/#find_nth/etc after
calling #reload or #reset on an association could return a stale record.
This is an issue both when the full association target is loaded and
when the item is loaded in #find_nth.

This commit solves the problem by clearing the `@offsets` cache in
CollectionProxy#reset and CollectionProxy#reload.

`ActiveRecord::Persistence#touch` does not work well when optimistic
locking enabled and `locking_column`, without default value, is null in
the database.

Fix destroying existing object does not work well when optimistic
locking enabled and `locking column` is null in the database.

Follow 22a822e5, #28914

Improves the performance from O(n) to O(1).
Previously it would require 50 queries to
insert 50 fixtures. Now it takes only one query.

Disabled on sqlite which doesn't support multiple inserts.