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.

The receiver in a scope should be a `relation`

Remove unused `Subject` model in tests

Use `timestamp_attributes_for_update_in_model` rather than `timestamp_attributes_for_update`

Remove `HasAndBelongsToManyReflection#initialize`

`Relation#locked?` should not build arel

Show query cache keys in `test_middleware_caches`

Fix eager loading to respect `store_full_sti_class` setting

Don't passing `klass.connection` to `AssociationScope`

Add a test case for unscoping `default_scope` in associations

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

Enable query cache if set a configurations

`test_middleware_caches` also failed same as #29600.

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

Fix grammar for documentation of rails/command.rb [ci skip]

Fix `ids_reader` to respect case sensitive primary key

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`.

Use `reload` in `test_find_first_after_reload`

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

Fixes #20679.
Closes #16531.

Show query cache keys in `test_exceptional_middleware_clears_and_disables_cache_on_error`

Add source code and changelog links to gemspecs

Remove useless class checking for `ActiveSupport::Callbacks`s result_lambda

[ci skip]

```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]]
```

ActiveRecord: do not create "has many through" records that have been removed

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

Fix missing formats in route-set URLs

Fix typo in callback deprecation message

Add more explicit guidance on how to fix callback deprecations

Calling `follow_redirect!` does not reset the `html_document`:

Extract `ordered_relation` in `FinderMethods`

Refactor join dependency to move building constraints to `join_scope` in `Reflection`

Document ActiveRecord's PostgreSQL-specific support for JSONB datatype