It should not be used in applications

This allows these exceptions to be handled generically in conjunction with RecordInvalid.

If you run a generator such as:

```
rails generate model accounts supplier:references
```

The resulting migration will now add the corresponding foreign key
constraint unless the reference was specified to be polymorphic.

Not sure how we missed this case when we moved everything else to the
`_quote` method.

The records weren't being replaced since equality in Active Record is
defined in terms of `id` only. It is reasonable to expect that the
references would be replaced in memory, even if no queries are actually
executed. This change did not appear to affect any other parts of the
code base. I chose not to execute callbacks since we're not actually
modifying the association in a way that will be persisted.

Fixes #17730

This reverts deprecations added in #13528.
The task is brought back for two reasons:
  1. Give plugins a way to hook into the test database initialization process
  2. Give the user a way to force a test database synchronization

While `test:prepare` is still a dependency of every test task, `db:test:prepare`
no longer hooks into it. This means that `test:prepare` runs before the schema
is synchronized. Plugins, which insert data can now hook into `db:test:prepare`.

The automatic schema maintenance can't detect when a migration is rolled-back,
modified and reapplied. In this case the user has to fall back to `db:test:prepare`
to force the synchronization to happen.

[Rafael Mendonça França & Yves Senn]

This require caused the `active_record.set_configs` initializer to
run immediately, before `config/initializers`. This means that setting any
configuration on `Rails.application.config.active_record` inside of
an initializer had no effects when rails was loaded through `rake`.

Introduced by #6518

/cc @rafaelfranca

This refactoring reduces the number of conditionals needed to build
`aliased_table_for` and removes `aliased_name_for` because it's no
longer necessary.

`aliased_name_for` was also used in `JoinDependency#initialize` so
that was replaced with `aliased_table_for` as well.

This allows us so abstract the migration from the type that is actually
used by Rails.  For example, ":string" may be a varchar or something,
but the framework does that translation, and the app shouldn't need to
know.

Some comments that are meant to separate blocks of code in a file show up
on http://api.rubyonrails.org as though they were part of the documentation.

This commit hides those comments from the documentation.

Stems from the discussion with @zzak at https://github.com/voloko/sdoc/issues/79#issuecomment-64158738

[ci skip]

Thanks to @thedarkone for pointing out that an instance of this object
is used in a shared context.

It now contains a carefully formulated reference to the "current relation" which might help clarify that the receiving will generate its own scope, escaping the need for explicitly referencing `default_scope` which is, after all, just another way of specifying a scope and nothing special.

For some reason changing `.find` to `.unscoped.find` in
https://github.com/rails/rails/commit/f766abd4cf3eb75469d3646cfb6d85e668c619f3
caused `scoping` to leak in some tests when run in isolation (looks like
a concurrency issue?). `relation_scoping_test.rb` is a case that failed.
From what I can tell it should not be possible, but changing to the
block form fixes it. There is a deeper issue that I can't seem to find.
/cc @senny

Also checked to make sure this does not affect foreign key constraints.
(It doesn't).

Fixes #12856
Closes #14088

We can't change the signature without a deprecation cycle.

on the joined assoiciation

Prior to this patch you'd end up with an error like:

```
ActiveRecord::RecordNotFound: Couldn't find <Model> with 'id'=<id> [WHERE (<default_scope condition>)]
```

Technically changes the API, as it will allow any object which responds
to `===`. Personally, I think this is more flexible.

I grepped the source code for code snippets wrapped in backticks in the comments
and replaced the backticks with plus signs so they are correctly displayed in
the Rails documentation.

[ci skip]

This makes the following changes:
  * warn if `:null` is not passed to `add_timestamps`
  * `timestamps` method docs link to `add_timestamps` docs
  * explain where additional options go
  * adjust examples to include `null: false` (to prevent deprecation warnings)

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.

It doesn't make sense for the subclass to implement this method, and not
have it on the parent. We can also DRY up the implementation of
`#lookup` to be defined in terms of fetch, which will give us a single
point of entry

This reverts commit da99a2a2.

We added a comparison to "id", and call to `self.class.primary_key` a
*lot*. We also have performance hits from `&block` all over the place.
We skip the check in a new method, in order to avoid breaking the
behavior of `read_attribute`

We don't have the check the range when the value is coming from the DB,
so override type_cast_from_database to short-circuit the extra work.
The difference is huge but the absolute gain is quite small. That being
said this is a hotspot and it showed up on the radar when benchmarking
discourse.

This appears to be a performance hotspot, see #17655.

Performance improvement, as well as improved code clarity