Previously we would spawn one thread per connection pool, which ends up
being wasteful for apps with several connection pools.

Fixes #34456.

This partly reverts the effect of d1107f4d.

d1107f4d makes `touch` tracks the mutation whether the `touch` is
occurred by explicit or not.

Existing apps expects that the previous changes tracks only the changes
which is explicit action by users.

I'd revert the implicit `touch` mutation tracking since I'd not like to
break existing apps.

Fixes #36219.

Since d1a74c1e, Active Record requires
SQLite version 3.8.0 or greater, so savepoints and partial indexes are
always available.

That commit also added a runtime version check, so we can remove the
minimum version requirement from the internal adapter documentation.

This commit adds "TRANSACTION" to savepoint and commit, rollback statements
because none of savepoint statements were removed by #36153 since they are not "SCHEMA" statements.

Although, only savepoint statements can be labeled as "TRANSACTION"
I think all of transaction related method should add this label.

Follow up #36153

The initializer receives `nil` for these options when no cofigurations were given:
https://github.com/rails/rails/blob/v6.0.0.rc1/activerecord/lib/active_record/railtie.rb#L91-L97

Currently, `committed!`/`rolledback!` will only be attempted for the
first enrolled record in the transaction, that will cause some
problematic behaviors.

The first one problem, `clear_transaction_record_state` won't be called
even if the transaction is finalized except the first enrolled record.
This means that de-duplicated records in the transaction won't refer
latest state (e.g. won't happen rolling back record state).

The second one problem, the enrolled order is not always the same as the
order in which the actions actually happened, the first enrolled record
may succeed no actions (e.g. `destroy` has already succeeded on another
record during `before_destroy`), it will lose to fire any transactional
callbacks.

To avoid both problems, we should attempt `committed!`/`rolledback!` to
all enrolled records in the transaction.

* Make scope arity check consistent

* Add test for arity change

[Rob Trame + Rafael Mendonça França]

If the same id's records are saved and/or destroyed in the transaction,
commit callbackes will only run for the first enrolled record.

https://github.com/rails/rails/blob/a023e2180093ebc517a642aaf21f3c7241c67657/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb#L115-L119

The regression #36132 is caused due to #35920 changed the enrollment
order that the first action's record will be enrolled to last in the
transaction.

We could not change the the enrollment order as long as someone depends
on the enrollment order.

Fixes #36132.

This fixes a regression for #35864.

Usually, stashed joins (mainly eager loading) are performed as LEFT
JOINs.
But the case of merging joins/left_joins of different class, that
(stashed) joins are performed as the same `join_type` as the parent
context for now.
Since #35864, both (joins/left_joins) stashed joins might be contained
in `joins_values`, so each stashed joins should maintain its own
`join_type` context.

Fixes #36103.

If we want to get alias resolved attribute finally, we can use
`attribute_alias` directly.

For that purpose, avoiding redundant `attribute_alias?` makes alias
attribute access 40% faster.

https://gist.github.com/kamipo/e427f080a27b46f50bc508fae3612a0e

Before (2c0729d8):

```
Warming up --------------------------------------
          user['id']   102.668k i/100ms
      user['new_id']    80.660k i/100ms
        user['name']    99.368k i/100ms
    user['new_name']    81.626k i/100ms
Calculating -------------------------------------
          user['id']      1.431M (± 4.0%) i/s -      7.187M in   5.031985s
      user['new_id']      1.042M (± 4.2%) i/s -      5.243M in   5.039858s
        user['name']      1.406M (± 5.6%) i/s -      7.055M in   5.036743s
    user['new_name']      1.074M (± 3.6%) i/s -      5.387M in   5.024152s
```

After (this change):

```
Warming up --------------------------------------
          user['id']   109.775k i/100ms
      user['new_id']   103.303k i/100ms
        user['name']   105.988k i/100ms
    user['new_name']    99.618k i/100ms
Calculating -------------------------------------
          user['id']      1.520M (± 6.7%) i/s -      7.574M in   5.011496s
      user['new_id']      1.485M (± 6.2%) i/s -      7.438M in   5.036252s
        user['name']      1.538M (± 5.4%) i/s -      7.737M in   5.049765s
    user['new_name']      1.516M (± 4.6%) i/s -      7.571M in   5.007293s
```

Follow up of #35838.

And also this refactors `in_clause_length` handling is entirely
integrated in Arel visitor.

* Avoid duplicated `@new_record` assignment
* Extract `define_attribute_methods` into `init_internals`

Follow up to b1458218.

Related 0ee96d13.

Those helper methods makes relation values access 15% slower.

https://gist.github.com/kamipo/e64439f7a206e1c5b5c69d92d982828e

Before (02b5b8cb):

```
Warming up --------------------------------------
        #limit_value   237.074k i/100ms
    #limit_value = 1   222.052k i/100ms
Calculating -------------------------------------
        #limit_value      6.477M (± 2.9%) i/s -     32.479M in   5.019475s
    #limit_value = 1      5.297M (± 4.3%) i/s -     26.424M in   4.999933s
```

After (this change):

```
Warming up --------------------------------------
        #limit_value   261.109k i/100ms
    #limit_value = 1   239.646k i/100ms
Calculating -------------------------------------
        #limit_value      7.412M (± 1.6%) i/s -     37.077M in   5.003345s
    #limit_value = 1      6.134M (± 1.0%) i/s -     30.675M in   5.000908s
```

I've realized that `user.id` is 20% slower than `user.name` in the
benchmark (https://github.com/rails/rails/pull/35987#issuecomment-483882480).

The reason that performance difference is that `self.class.primary_key`
method call is a bit slow.

Avoiding that method call will make almost attribute access faster and
`user.id` will be completely the same performance with `user.name`.

Before (02b5b8cb):

```
Warming up --------------------------------------
             user.id   140.535k i/100ms
          user['id']    96.549k i/100ms
           user.name   158.110k i/100ms
        user['name']    94.507k i/100ms
       user.changed?    19.003k i/100ms
 user.saved_changes?    25.404k i/100ms
Calculating -------------------------------------
             user.id      2.231M (± 0.9%) i/s -     11.243M in   5.040066s
          user['id']      1.310M (± 1.3%) i/s -      6.565M in   5.012607s
           user.name      2.683M (± 1.2%) i/s -     13.439M in   5.009392s
        user['name']      1.322M (± 0.9%) i/s -      6.615M in   5.003239s
       user.changed?    201.999k (±10.9%) i/s -      1.007M in   5.091195s
 user.saved_changes?    258.214k (±17.1%) i/s -      1.245M in   5.007421s
```

After (this change):

```
Warming up --------------------------------------
             user.id   158.364k i/100ms
          user['id']   106.412k i/100ms
           user.name   158.644k i/100ms
        user['name']   107.518k i/100ms
       user.changed?    19.082k i/100ms
 user.saved_changes?    24.886k i/100ms
Calculating -------------------------------------
             user.id      2.768M (± 1.1%) i/s -     13.936M in   5.034957s
          user['id']      1.507M (± 2.1%) i/s -      7.555M in   5.017211s
           user.name      2.727M (± 1.5%) i/s -     13.643M in   5.004766s
        user['name']      1.521M (± 1.3%) i/s -      7.634M in   5.018321s
       user.changed?    200.865k (±11.1%) i/s -    992.264k in   5.044868s
 user.saved_changes?    269.652k (±10.5%) i/s -      1.344M in   5.077972s
```

It was never used from the beginning.

Method call in Ruby is a bit slow.

This makes attribute access 10% faster by avoiding method call
(`sync_with_transaction_state`).

Before (96cf7e0e):

```
Warming up --------------------------------------
             user.id   131.291k i/100ms
          user['id']    91.786k i/100ms
           user.name   151.605k i/100ms
        user['name']    92.664k i/100ms
       user.changed?    17.772k i/100ms
 user.saved_changes?    23.909k i/100ms
Calculating -------------------------------------
             user.id      1.988M (± 7.0%) i/s -      9.978M in   5.051474s
          user['id']      1.155M (± 5.8%) i/s -      5.783M in   5.022672s
           user.name      2.450M (± 4.3%) i/s -     12.280M in   5.021234s
        user['name']      1.263M (± 2.1%) i/s -      6.394M in   5.066638s
       user.changed?    175.070k (±13.3%) i/s -    853.056k in   5.011555s
 user.saved_changes?    259.114k (±11.8%) i/s -      1.267M in   5.001260s
```

After (this change):

```
Warming up --------------------------------------
             user.id   137.625k i/100ms
          user['id']    96.054k i/100ms
           user.name   156.379k i/100ms
        user['name']    94.795k i/100ms
       user.changed?    18.172k i/100ms
 user.saved_changes?    24.337k i/100ms
Calculating -------------------------------------
             user.id      2.201M (± 0.5%) i/s -     11.010M in   5.002955s
          user['id']      1.320M (± 1.0%) i/s -      6.628M in   5.021293s
           user.name      2.677M (± 1.6%) i/s -     13.449M in   5.024399s
        user['name']      1.314M (± 1.8%) i/s -      6.636M in   5.051444s
       user.changed?    190.588k (±11.1%) i/s -    944.944k in   5.065848s
 user.saved_changes?    262.782k (±12.1%) i/s -      1.290M in   5.028080s
```

This was accidentally left in, the standard `db:migrate:up` doesn't have
a description so `db:migrate:up:namespace` shouldn't have one either.

This change adds the ability to run up/down for a database in a multi-db
environment.

If you have an app with a primary and animals database the following
tasks will be generated:

```
VERSION=123 rake db:migrate:up:primary
VERSION=123 rake db:migrate:up:primary

VERSION=123 rake db:migrate:down:primary
VERSION=123 rake db:migrate:up:animals
```

I didn't generate descriptions with them since we don't generate a
description for a single database application.

In addition to this change I've made it so if your application has
multiple databases Rails will raise if you try to run `up` or `down`
without a namespace. This is because we don't know which DB you want to
run `up` or `down` against unless the app tells us, so it's safer to
just block it and recommend using namespaced versions of up/down
respectively.

The output for the raise looks like:

```
You're using a multiple database application. To use `db:migrate:down`
you must run the namespaced task with a VERSION. Available tasks are
db:migrate:down:primary and db:migrate:down:animals.
```

This reverts commit f656bb30.

Reason: Test in Action View expects the `collection_cache_key` working...

https://github.com/rails/rails/blob/ff6b713f5e729859995f204093ad3f8e08f39ea8/actionview/test/activerecord/relation_cache_test.rb#L21
https://github.com/rails/rails/blob/ff6b713f5e729859995f204093ad3f8e08f39ea8/actionview/test/fixtures/project.rb#L6

https://buildkite.com/rails/rails/builds/60609#d19181fb-fe80-4d1e-891c-1109b540fb4b/981-1009

The `collection_cache_key` is private API for a long time, but I've
maintained it in #35848 since it is mentioned in the doc
(https://github.com/rails/rails/pull/35848#discussion_r272011475).

The doc has removed at 1da9a7e4, so there is no longer a reason to
maintain that private API.

This follows #35595.

This partly reverts 8ca3c286.

`.is_a?(GeneratedAttributeMethodsBuilder)` doesn't make sense to me, the
module class's `new` creates just an instance of the module class itself.

Actionable errors let's you dispatch actions from Rails' error pages. This
can help you save time if you have a clear action for the resolution of
common development errors.

The de-facto example are pending migrations. Every time pending migrations
are found, a middleware raises an error. With actionable errors, you can
run the migrations right from the error page. Other examples include Rails
plugins that need to run a rake task to setup themselves. They can now
raise actionable errors to run the setup straight from the error pages.

Here is how to define an actionable error:

```ruby
class PendingMigrationError < MigrationError #:nodoc:
  include ActiveSupport::ActionableError

  action "Run pending migrations" do
    ActiveRecord::Tasks::DatabaseTasks.migrate
  end
end
```

To make an error actionable, include the `ActiveSupport::ActionableError`
module and invoke the `action` class macro to define the action. An action
needs a name and a procedure to execute. The name is shown as the name of a
button on the error pages. Once clicked, it will invoke the given
procedure.

`where.not` with polymorphic association is partly fixed incidentally at
213796fb (refer #33493, #26207, #17010, #16983, #14161), and I've added
test case e9ba12f7 to avoid lose that fix accidentally in the future.

In Rails 5.2, `where.not(polymorphic: object)` works as expected as
NAND, but `where.not(polymorphic_type: object.class.polymorphic_name,
polymorphic_id: object.id)` still unexpectedly works as NOR.

To will make `where.not` working desiredly as NAND in Rails 6.1, this
deprecates `where.not` working as NOR. If people want to continue NOR
conditions, we'd encourage to them to `where.not` each conditions
manually.

```ruby
all = [treasures(:diamond), treasures(:sapphire), cars(:honda), treasures(:sapphire)]
assert_equal all, PriceEstimate.all.map(&:estimate_of)
```

In Rails 6.0:

```ruby
sapphire = treasures(:sapphire)

nor = all.reject { |e|
  e.estimate_of_type == sapphire.class.polymorphic_name
}.reject { |e|
  e.estimate_of_id == sapphire.id
}
assert_equal [cars(:honda)], nor

without_sapphire = PriceEstimate.where.not(
  estimate_of_type: sapphire.class.polymorphic_name, estimate_of_id: sapphire.id
)
assert_equal nor, without_sapphire.map(&:estimate_of)
```

In Rails 6.1:

```ruby
sapphire = treasures(:sapphire)

nand = all - [sapphire]
assert_equal [treasures(:diamond), cars(:honda)], nand

without_sapphire = PriceEstimate.where.not(
  estimate_of_type: sapphire.class.polymorphic_name, estimate_of_id: sapphire.id
)
assert_equal nand, without_sapphire.map(&:estimate_of)
```

Resolves #31209.