Deprecate leaking scope in callback block for association relation

Maintain extra joins for string or complex arel conditions

Include connection in cached query notifications

Better double checked locking in load_schema

Rollback in `after_commit` should not rollback state that already been succeeded

Maintain eager loading joining order as before

Fix error message for AdapterNotFound in spec

Clear query cache when insert all is used

Allow inspected records to be marshaled

Extend `test_can_write_while_reading_from_replicas_if_explicit`

Call `while_preventing_writes` from `connected_to`

Address to "DEPRECATION WARNING: Uniqueness validator will no longer enforce case sensitive comparison in Rails 6.1"

Caused by #36210.

Improve MySQL error detection

Fix error on reap/flush for closed connection pool

Fix setting connection_specification_name on ActiveRecord::Base

Ensure connection reaper threads respawn in forks

Clear ActiveRecord object memoized by take

Make prepared statement status thread and instance-specific

Raise TypeError instead of infinite loop in resolve_connection

Actually, `first` taking non-deterministic result is considered a bug to
me. But changing the behavior should not be happened in rc2.

I've restored the behavior for 6.0, and then will deprecate that in 6.1.

Fixes #36802.

Fix GROUP BY aggregation alias to not duplicate "_" chars

Fix thread safety of prevent_writes

Add ability to unset preventing writes

Introduce InvalidConfigurationError

Deduplicate joins values

To avoid a deprecation warning.

Preserve user supplied joins order as much as possible

To suppress the following warnings in tests.

```
~/rails/activerecord/lib/active_record/scoping/named.rb:190: warning: method redefined; discarding old not_sent
~/rails/activerecord/lib/active_record/scoping/named.rb:190: warning: previous definition of not_sent was here
```

Allow separate database env variables per-connection

Fix join middle table alias when using HABTM

Only merge DATABASE_URL settings into the current env

Use connection.error_number in MySQLDatabaseTasks

Add a warning for enum elements with 'not_' prefix.

Fix index_exists? for PostgreSQL expression indexes

Fix multiple database support for DATABASE_URL env variable

Don't validate non dirty association targets