Apart from specific versioning support, our tests should focus on the
behaviour of whatever version they're accompanying, regardless of when
they were written.

Application code should *not* do this.

If we use a real version, at best that'll be an onerous update required
for each release; at worst, it will encourage users to write new
migrations against an older version than they're using.

The other option would be to leave these bare, without any version
specifier. But as that's just a variant spelling of "4.2", it would seem
to raise the same concerns as above.

This reverts commit 6d5b1fdf.

`eager_load` and `references` can include hashes, which won't match up
with `references`

A test case has been added to demonstrate the problem

This was changed in 421c81bd, as `exists?` blows up if you are eager
loading a polymorphic association, as it'll try to construct a join to
that table. The previous change decided to execute a `count` instead,
which wouldn't join.

Of course, the only time we actually need to perform a join on the eager
loaded values (which would perform a left outer join) is if they're
being referenced in the where clause. This doesn't affect inner joins.

There was a test remaining for PG only that was checking for an exact
limit clause

We currently generate an unbounded number of prepared statements when
`limit` or `offset` are called with a dynamic argument. This changes
`LIMIT` and `OFFSET` to use bind params, eliminating the problem.

`Type::Value#hash` needed to be implemented, as it turns out we busted
the query cache if the type object used wasn't exactly the same object.

This drops support for passing an `Arel::Nodes::SqlLiteral` to `limit`.
Doing this relied on AR internals, and was never officially supported
usage.

Fixes #22250.

Some backends allow `LIMIT 1,2` as a shorthand for `LIMIT 1 OFFSET 2`.
Supporting this in Active Record massively complicates using bind
parameters for limit and offset, and it's trivially easy to build an
invalid SQL query by also calling `offset` on the same `Relation`.

This is a niche syntax that is only supported by a few adapters, and can
be trivially worked around by calling offset explicitly.

This reverts commit c2e70ca9, reversing
changes made to b0e5fc27.

this broke the build

In the doc the `dependent` option was set with: `dependent: destroy`.
This is not working because destroy would call the method of the activerecord::base object.

The right way is: `dependent: :destroy`

The problem was that when saving an object, we would
call touch_later on the parent which wont be saved immediteally, and
it wont call any callbacks. That was working one level up because
we were calling touch, during the touch_later commit phase. However that still
didnt solve the problem when you have a 3+ levels of parents to be touched,
as calling touch would affect the parent, but it would be too late to run callbacks
on its grand-parent.

The solution for this, is instead, call touch_later upwards when the first
touch_later is called. So we make sure all the timestamps are updated without relying
on callbacks.

This also removed the hard dependency BelongsTo builder had with the TouchLater module.
So we can still have the old behaviour if TouchLater module is not included.

[fixes 5f5e6d92]
[related #19324]

Those are actually shortcuts for `after_commit`.

Before:

    after_commit :add_to_index_later, on: :create
    after_commit :update_in_index_later, on: :update
    after_commit :remove_from_index_later, on: :destroy

After:

    after_create_commit  :add_to_index_later
    after_update_commit  :update_in_index_later
    after_destroy_commit :remove_from_index_later

According to pr #22443 in the guides there's always a dollar sign before every command, so why is in the main README a `$` and in every submodule a `%`?

Just eye candy..

The `rake db:test:*` tasks were deprecated in #13528, but were
undeprecated and added back in via #17739.

This removes the following warning which has been out in the case of a PostgreSQL 9.3 below.

```
activerecord/test/cases/adapters/postgresql/geometric_test.rb:265: warning: instance variable @connection not initialized
```

[ci skip]

Q: What happens if you initialize an AR model by passing Parameters that
have not been whitelisted with `permit`?

A: An `ActiveModel::ForbiddenAttributesError` is raised.

I think this behavior is correct, and it's better than what used to happen,
with unpermitted parameter being simply ignored.

This will also get the defaults from attribute definitions like:

     attribute :type, :string, default: "SomethingElse"

`has_attribute?` method to check wether a given attribute has been defined.

This solves the following error:

     ActiveRecord::StatementInvalid: Could not find table 'guitars'

It seems that the table structure of the `Guitar` model has not been
necessary until now. Due to the wrong table name the model was not
correctly linked to the table.

fixes #17121

It appears that I missed this one when I delegated all the non-mutation
array methods that were not on Enumerable

The previous message was misleading (especially for Ops guys) when
diagnosing problems related to the database connection.

The message was suggesting that the connection cannot be obtained which
normally assumes the need to look at the database.

But this isn't the case as the connection could not be retrieved from
the application's internal connection pool.

The new message should make it more explicit and remove the confusion.

`pool` in args is unused anymore. And `config` is used in all adapters.

Not needed for `Mysql2Adapter` and `AbstractMysqlAdapter`.

[ci skip]

Not needed for `Mysql2Adapter` and `AbstractMysqlAdapter`.

As was pointed out by #17128, our blacklist of mutation methods was
non-exhaustive (and would need to be kept up to date with each new
version of Ruby). Now that `Relation` includes `Enumerable`, the number
of methods that we actually need to delegate are pretty small. As such,
we can change to explicitly delegating the few non-mutation related
methods that `Array` has which aren't on `Enumerable`