`through_reflection` and `source_reflection` are used only in the class.

`associated_records_by_owner` had returned customizing result before
calling `associate_records_to_owner` for through association subclasses.
Since #22115, `associate_records_to_owner` is called in the method and
not returned owner and result pairs. Removing the method will reduce
method call and block call nesting.

Ensure that `bin/rails db:migrate` with specified `VERSION` reverts
all migrations only if `VERSION` is `0`.
Raise error if target migration doesn't exist.

```
[NoMethodError]: undefined method `state' for nil:NilClass  Method:[rescue in block in refresh]
```

In `within_new_transaction`, there is the possibility that
`begin_transaction` returns a `nil`. (i.e.: so `transaction = nil`)
So this method is checking `transaction` for nil in 2 spots.

Unfortunately, there is one line that is not checking `transaction` for `nil`
That line, `commit_transaction`, throws an exception for us in AR 5.0.0.1

The problem with the method is finally realized in the error checking itself.
it calls `transaction.state` (i.e.: nil.state) and that is the final exception
raised.

The actual underlying (user) issue is hidden by this line.

Solution is test transaction for nil.

A relation includes `klass`, so it can not be used as it is if current
class is different from `current_scope.klass`. It should be created new
relation by current class to respect the klass and STI constraint.

Fixes #17603.
Fixes #23576.

This is partially fixed by e617fb57 when through association has already
loaded. Otherwise, second level through association should respect
`preload_scope`.

Fixes #30242.
Closes #30076.

[Ryuta Kamizono & CicholGricenchos]

If through association has already loaded, `source_type` is ignored to
loaded through records. The loaded records should be filtered by
`source_type` in that case.

Fixes #30904.

If a relation has eager-loading values, `count` and `exists?` works
properly, but `update_all` and `delete_all` doesn't work due to missing
`apply_join_dependency`. It should be applied to work consistently.

Fixes #28863.

Fixes #30315.

* `HasManyThroughCantDissociateNewRecords` and `HasManyThroughCantAssociateNewRecords`
are no longer used since f6b12c11.
* `ReadOnlyAssociation` is no longer used since 0da426be.

`join_scopes` in `PolymorphicReflection` is passed aliased `table`, so
it should be respected for `source_type_scope`.

Closes #13969.
Fixes #13920.
Fixes #15190.

Replace the primary key type `integer` in docs with `bigint`.

ref #26266

using serializable isolation would prevent the duplicate insert as done in the example from happening

and abstract `SchemaDumper#extensions` is now an empty method.

Since #30337, every database adapter has its own `SchemaDumper`.
`extensions` are only supported by PostgreSQL database and postgresql database adapter.

since the minimum version of PostgreSQL currently Rails supports is 9.1,
there is no need to handle if `supports_extensions?`

Refer https://www.postgresql.org/docs/9.1/static/sql-createextension.html
"CREATE EXTENSION"

Although the sqlite adapter supports index sort orders, they
weren't being written to db/schema.rb.

Currently alias tracker only refer a table name, doesn't respect an
alias name. Should use `join.left.name` rather than
`join.left.table_name`.

`supports_disable_referential_integrity?` used to handle
if PostgreSQL database supports
`ALTER TABLE <table name> DISABLE/ENABLE TRIGGER` statements.
Refer https://github.com/rails/rails/commit/9a947af0e79cfb8692eb7e5ae94c1b8c40756f49

These statements have been documented since 8.1.

https://www.postgresql.org/docs/8.1/static/sql-altertable.html
> DISABLE/ENABLE TRIGGER

Now Rails supports PostgreSQL 9.1 or higher only.
No need to handle `supports_disable_referential_integrity?` anymore.
Also, this method does not exist in any other adapters including AbstractAdapter.

Remove deprecated methods `initialize_schema_migrations_table` and `initialize_internal_metadata_table`