We should call methods with `.method_name` not `::method_name`. Fix two
instances of `YAML::load` I found in favor of `YAML.load`.

Because of this default configuration we're constantly checking if the
database exists when looping through configurations. This is unnecessary
and we should just delete it before we need to loop through
configurations.

Fixes #32021.

3acc5d6e was changed the order of scope evaluation from through scope to
the association's own scope to be prioritized over the through scope.
But the sorting order will be prioritized that is evaluated first. It is
unintentional effect, association scope's sorting order should be
prioritized as well.

Fixes #32008.

This reverts commit cf4f05a7.

Since Rails 6 requires Ruby 2.4.1+.

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Closes #31998

Some attr_readers should be `protected` instead of `private`

See https://travis-ci.org/rails/rails/builds/342800276

References 89bcca59

References 89bcca59

Some places we can't remove because Ruby still don't have a method
equivalent to strip_heredoc to be called in an already existent string.

Provides both a forked process and threaded parallelization options. To
use add `parallelize` to your test suite.

Takes a `workers` argument that controls how many times the process
is forked. For each process a new database will be created suffixed
with the worker number; test-database-0 and test-database-1
respectively.

If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
and the environment variable will be used instead. This is useful for CI
environments, or other environments where you may need more workers than
you do for local testing.

If the number of workers is set to `1` or fewer, the tests will not be
parallelized.

The default parallelization method is to fork processes. If you'd like to
use threads instead you can pass `with: :threads` to the `parallelize`
method. Note the threaded parallelization does not create multiple
database and will not work with system tests at this time.

parallelize(workers: 2, with: :threads)

The threaded parallelization uses Minitest's parallel exector directly.
The processes paralleliztion uses a Ruby Drb server.

For parallelization via threads a setup hook and cleanup hook are
provided.

```
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end

  parallelize_teardown do |worker|
    # cleanup database
  end

  parallelize(workers: 2)
end
```

[Eileen M. Uchitelle, Aaron Patterson]

Add #create_or_find_by to lean on unique constraints

[ci skip]

* Add Relation#pick as short-hand for single-value plucks

This reverts commit 487a1061.

This `#--` is necessary for the doc of `distinct`.

[ci skip]

Currently `_default_attributes` doesn't work unless `load_schema` is
called before.

The `MissingAttributeError` is caused by `reload_schema_from_cache` is
invoked by `serialize`.

I added `load_schema` in `_default_attributes` to `_default_attributes`
works without any dependency like `attribute_types` etc.

Closes #31905.

Followup to b988ecb9, when I cherry-picked from my parallel testing
branch I didn't realize `drop` wasn't part of the commit.

You could use the `VERBOSE` env var to turn off output for migrations
tasks but you couldn't use it for other tasks.

This change moves the `verbose?` check to a method so we can also use it
in create and drop respectively.

tenderlove and I noticed this as part of the ongoing work in parallel
testing. When the parallel tests boot the app needs to create new
databases for each worker. The output from these is unnecessary but
there was previously no way to turn it off. Now if `VERBOSE=false` is
passes to `bin/rails db:create` the text "Created blah blah db" will no
longer be output.

Prefer `@connection.abandon_results!` than `@connection.next_result while @connection.more_results?`

These are internally used only.

[ci skip]

:tada::tada::tada:

This can not expand an array of aggregated objects correctly, and is no
longer used internally since 159b21b5.

With #31615 `type_for_attribute` accepts either
a symbol as well as a string. `has_attribute?` and `attribute_alias`
also accept either. Since these methods call `to_s` on the argument,
we no longer need to do that at the call site.

This example was added in abdf546a
but was inconsistent with `composed_of :balance` definition in the
'Customer'.

[Ryuta Kamizono & bogdanvlviv]

[ci skip]

This reverts commit 755f7b58, reversing
changes made to 35db4603.

Originally `mapping: %w(balance amount)` is correct mapping example.

Closes #31346.

[Ryuta Kamizono & bogdanvlviv]

[ci skip]

Follow up of #31724.

If `composed_of` objects have multiple mappings, array predicate handler
can not correctly handle the expanded condition.

We need to handle it like polymorphic association objects.

Since #29504, mysql2 adapter lost ability to insert zero value on
primary key due to enforce `NO_AUTO_VALUE_ON_ZERO` disabled.
That is for using `DEFAULT` on auto increment column, but we can use
`NULL` instead in that case.