[ci skip]

These extra `spawn` are called via `klass.all` and `klass.all` is called
everywhere in the internal. Avoiding the extra `spawn` makes` klass.all`
30% faster for STI classes.

https://gist.github.com/kamipo/684d03817a8115848cec8e8b079560b7

```
Warming up --------------------------------------
       fast relation     4.410k i/100ms
       slow relation     3.334k i/100ms
Calculating -------------------------------------
       fast relation     47.373k (± 5.2%) i/s -    238.140k in   5.041836s
       slow relation     35.757k (±15.9%) i/s -    176.702k in   5.104625s

Comparison:
       fast relation:    47373.2 i/s
       slow relation:    35756.7 i/s - 1.32x  slower
```

Use whatever adapter-provided means we have available to ensure forked
children don't send quit/shutdown/goodbye messages to the server on
connections that belonged to their parent.

Currently `_create_record` and `_update_record` in `Persistence` are
creating extra `unscoped` and calling `build_arel` in the relation. But
`compile_insert` and `compile_update` can be done without those
expensive operation for `SelectManager` creation. So I moved the
implementation to `Persistence` to avoid creating extra relation and
refactored to avoid calling `build_arel`.

https://gist.github.com/kamipo/8ed73d760112cfa5f6263c9413633419

Before:

```
Warming up --------------------------------------
      _update_record   150.000  i/100ms
Calculating -------------------------------------
      _update_record      1.548k (±12.3%) i/s -      7.650k in   5.042603s
```

After:

```
Warming up --------------------------------------
      _update_record   201.000  i/100ms
Calculating -------------------------------------
      _update_record      2.002k (±12.8%) i/s -      9.849k in   5.027681s
```

30% faster for STI classes.

* Add a #populate method to migrations

* Address rubocop issues

* Rename to #up_only and use #execute in the examples intead of the model

* Update CHANGELOG

[Rich Daley & Rafael Mendonça França]

Make it clear that `exists?` can be chained onto a relation

Currently the environment is not loaded in some db tasks.
Therefore, if use encrypted secrets values in `database.yml`,
`read_encrypted_secrets` will not be true, so the value can not be
used correctly.

To fix this, added `environment` as dependency of `load_config`.
It also removes explicit `environment` dependencies that are no longer
needed.

Fixes #30717

The documentation claims that given values go through "normal AR type
casting and serialization", which to me implies
`serialize(cast(value))`, not just serialization. The docs were changed
to use this wording in #22492. The tests I cited in that PR (which is
the same test modified in this commit), is worded in a way that implies
it should be using `cast` as well.

It's possible that I originally meant "normal type casting" to imply
just the call to `serialize`, but given that `update_all(archived:
params['archived'])` seems to be pretty common, I'm inclined to make
this change as long as no tests are broken from it.

If there are any ignored columns, we will now list out all columns we
want to be returned from the database.

Includes a regression test.

Add new error class `StatementTimeout` which will be raised when statement timeout exceeded (#31129)

We are sometimes using The MAX_EXECUTION_TIME hint for MySQL depending
on the situation. It will prevent catastrophic performance down by wrong
performing queries.

The new error class `StatementTimeout` will make to be easier to handle
that case.

https://dev.mysql.com/doc/refman/5.7/en/optimizer-hints.html#optimizer-hints-execution-time

Follow up of #30360.

Attribute modules (`Attribute`, `Attributes`, `AttributeSet`) uses
`Type`, but referencing `Type` before the modules still fail.

```
% ./bin/test -w test/cases/attribute_test.rb -n test_with_value_from_user_validates_the_value
Run options: -n test_with_value_from_user_validates_the_value --seed 31876

E

Error:
ActiveModel::AttributeTest#test_with_value_from_user_validates_the_value:
NameError: uninitialized constant ActiveModel::AttributeTest::Type
    /Users/kamipo/src/github.com/rails/rails/activemodel/test/cases/attribute_test.rb:233:in `block in <class:AttributeTest>'

bin/test test/cases/attribute_test.rb:232

Finished in 0.002985s, 335.0479 runs/s, 335.0479 assertions/s.
1 runs, 1 assertions, 0 failures, 1 errors, 0 skips
```

Probably we need more autoloading at least `Type`.

`joins_values.partition` will break joins values order. It should be
kept as user intended order.

Fixes #15488.

Fixes #21577.

They are only different by one line of code which doesn't deserve a
hierarchy of 7 classes.

Closes #31079.

[Ryuta Kamizono & Bogdan Gusiev]

Use these to back the attributes API.  Stop automatically including
ActiveModel::Dirty in ActiveModel::Attributes, and make it optional.

This fixes an issue where competing threads deadlock each other.

- Thread A holds the load interlock but is blocked on getting the DB lock
- Thread B holds the DB lock but is blocked on getting the load interlock (for example when there is a `Model.transaction` block that needs to autoload)

This solution allows for dependency loading in other threads while a thread is waiting to acquire the DB lock.

Fixes #31019

use << instead of #concat in #reverse_sql_order because we might be working with Arel SQL literator which overrides #concat

`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]