Follow up #32146.

`init_with` and `init_from_db` are almost the same code except decode
`coder`.

And also, named `init_from_db` is a little misreading, a raw values hash
from the database is already converted to an attributes object by
`attributes_builder.build_from_database`, so passed `attributes` in that
method is just an attributes object.

I renamed that method to `init_with_attributes` since the method is
shared with `init_with` to initialize an empty model object.

I don't prefer to extract it for one adapter even though all adapters
also does.

Related to #34227.

Will allow sub classes to override the protected
`#check_version` method hook if desired.

For example, this will be most helpful in sub classes that wish
to support lazy initialization because the version check can
be postponed until the connection is ready to be initialized.

Change query to use alias name for timestamp_column to avoid ambiguity problems when using timestamp from subquery.

Ruby uses the original method name, so will show the __temp__ method
name in the backtrace. However, in the common case the method name
is compatible with the `def` keyword, so we can avoid the __temp__
method name in that case to improve the name shown in backtraces
or TracePoint#method_id.

This PR adds the ability to 1) connect to multiple databases in a model,
and 2) switch between those connections using a block.

To connect a model to a set of databases for writing and reading use
the following API. This API supercedes `establish_connection`. The
`writing` and `reading` keys represent handler / role names and
`animals` and `animals_replica` represents the database key to look up
the configuration hash from.

```
class AnimalsBase < ApplicationRecord
  connects_to database: { writing: :animals, reading: :animals_replica }
end
```

Inside the application - outside the model declaration - we can switch
connections with a block call to `connected_to`.

If we want to connect to a db that isn't default (ie readonly_slow) we
can connect like this:

Outside the model we may want to connect to a new database (one that is
not in the default writing/reading set) - for example a slow replica for
making slow queries. To do this we have the `connected_to` method that
takes a `database` hash that matches the signature of `connects_to`. The
`connected_to` method also takes a block.

```
AcitveRecord::Base.connected_to(database: { slow_readonly: :primary_replica_slow }) do
  ModelInPrimary.do_something_thats_slow
end
```

For models that are already loaded and connections that are already
connected, `connected_to` doesn't need to pass in a `database` because
you may want to run queries against multiple databases using a specific
role/handler.

In this case `connected_to` can take a `role` and use that to swap on
the connection passed. This simplies queries - and matches how we do it
in GitHub. Once you're connected to the database you don't need to
re-connect, we assume the connection is in the pool and simply pass the
handler we'd like to swap on.

```
ActiveRecord::Base.connected_to(role: :reading) do
  Dog.read_something_from_dog
  ModelInPrimary.do_something_from_model_in_primary
end
```

When defining a Hash enum it can be easy to use [] instead of {}. This
commit checks that only valid definition values are provided, those can
be a Hash, an array of Symbols or an array of Strings. Otherwise it
raises an ArgumentError.

Fixes #33961

The delegation methods to named scope are defined when `method_missing`
is invoked on the relation.

Since #29301, the receiver in the named scope is changed to the relation
like others (e.g. `default_scope`, etc) for consistency.

Most named scopes would be delegated from relation by `method_missing`,
since we don't allow scopes to be defined which conflict with instance
methods on `Relation` (#31179). But if a named scope is defined with the
same name as any method on the `superclass` (e.g. `Kernel.open`), the
`method_missing` on the relation is not invoked.

To address the issue, make the delegation methods to named scope is
generated in the definition time.

Fixes #34098.

[ci skip]

Since #31575, `set_inverse_instance` replaces the foreign key by the
current owner immediately to make it happen when a record is added to
collection association.

But `set_inverse_instance` is not only called when a record is added,
but also when a record is loaded from queries. And also, that loaded
records are not always associated records for some reason (using `or`,
`unscope`, `rewhere`, etc).

It is hard to distinguish whether or not we should invoke
`set_inverse_instance`, but at least we should avoid the undesired
side-effect which was brought from #31575.

Fixes #34108.

Since the above comment is for the `preloaders_for_one`.

`establish_connection` will never raise `ActiveRecord::NoDatabaseError`,
because it doesn't connect to a database; it sets up a connection pool.

MySQL supports DELETE with LIMIT and ORDER BY.

https://dev.mysql.com/doc/refman/8.0/en/delete.html

Before:

```
  Post Destroy (1.0ms)  DELETE FROM `posts` WHERE `posts`.`id` IN (SELECT `id` FROM (SELECT `posts`.`id` FROM `posts` WHERE `posts`.`author_id` = ? ORDER BY `posts`.`id` ASC LIMIT ?) __active_record_temp)  [["author_id", 1], ["LIMIT", 1]]
```

After:

```
  Post Destroy (0.4ms)  DELETE FROM `posts` WHERE `posts`.`author_id` = ? ORDER BY `posts`.`id` ASC LIMIT ?  [["author_id", 1], ["LIMIT", 1]]
```

Since Rails 6.0 will support Ruby 2.4.1 or higher
`# frozen_string_literal: true` magic comment is enough to make string object frozen.
This magic comment is enabled by `Style/FrozenStringLiteralComment` cop.

* Exclude these files not to auto correct false positive `Regexp#freeze`
 - 'actionpack/lib/action_dispatch/journey/router/utils.rb'
 - 'activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb'

It has been fixed by https://github.com/rubocop-hq/rubocop/pull/6333
Once the newer version of RuboCop released and available at Code Climate these exclude entries should be removed.

* Replace `String#freeze` with `String#-@` manually if explicit frozen string objects are required

 - 'actionpack/test/controller/test_case_test.rb'
 - 'activemodel/test/cases/type/string_test.rb'
 - 'activesupport/lib/active_support/core_ext/string/strip.rb'
 - 'activesupport/test/core_ext/string_ext_test.rb'
 - 'railties/test/generators/actions_test.rb'

When one relation is merged into another that has a different base class
merging `from_clause` causes invalid SQL to be generated

Before:

```
  Topic Update All (0.4ms)  UPDATE `topics` SET `topics`.`replies_count` = COALESCE(`topics`.`replies_count`, 0) + 1, `topics`.`updated_at` = '2018-09-27 18:34:05.068774' WHERE `topics`.`id` = ?  [["id", 7]]

```

After:

```
  Topic Update All (0.4ms)  UPDATE `topics` SET `topics`.`replies_count` = COALESCE(`topics`.`replies_count`, 0) + ?, `topics`.`updated_at` = ? WHERE `topics`.`id` = ?  [["replies_count", 1], ["updated_at", 2018-09-27 18:55:05 UTC], ["id", 7]]

```

Before:

```
  Pet Update All (0.8ms)  UPDATE `pets` LEFT OUTER JOIN `toys` ON `toys`.`pet_id` = `pets`.`pet_id` SET `pets`.`name` = 'Bob' WHERE `toys`.`name` = ?  [["name", "Bone"]]
```

After:

```
  Pet Update All (1.1ms)  UPDATE `pets` LEFT OUTER JOIN `toys` ON `toys`.`pet_id` = `pets`.`pet_id` SET `pets`.`name` = ? WHERE `toys`.`name` = ?  [["name", "Bob"], ["name", "Bone"]]
```

Since counter cache handles touch option too.

Refactor counter cache to extract `decrement_counters_before_last_save` on the belongs_to association