The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Actually schema dumper/creation supports composite primary key (#21614).
Therefore it should not show the warning about composite primary key in
connection adapter.

This change moves the warning to `AttributeMethods::PrimaryKey` and
suppress the warning for habtm join table.

Fixes #25388.

The focus of this change is to make the API more accessible.
References to method and classes should be linked to make it easy to
navigate around.

This patch makes exzessiv use of `rdoc-ref:` to provide more readable
docs. This makes it possible to document `ActiveRecord::Base#save` even
though the method is within a separate module
`ActiveRecord::Persistence`. The goal here is to bring the API closer to
the actual code that you would write.

This commit only deals with Active Record. The other gems will be
updated accordingly but in different commits. The pass through Active
Record is not completely finished yet. A follow up commit will change
the spots I haven't yet had the time to update.

/cc @fxn

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 know which attributes will or won't be used, and we don't want
to create massive bottlenecks at instantiation. Rather than doing *any*
iteration over types and values, we can lazily instantiate the object.

The lazy attribute hash should not fully implement hash, or subclass
hash at any point in the future. It is not meant to be a replacement,
but instead implement its own interface which happens to overlap.

Currently when we call id_was and we have a custom primary key name
Active Record will return the current value of the primary key. This
make impossible to correctly do an update operation if you change the
id.

Fixes #16413

Making this change revealed several subtle bugs related to models with
no primary key, and anonymous classes. These have been fixed as well,
with regression tests added.

We'd spend a lot of time calling `hash` and `eql?` on the join model,
which has no primary key. Calling `id` with no primary key is a really
slow way to get back `nil`, so we can improve the performance there.
However, even with the escape clause, we *still* weren't getting high
enough performance, as we were checking the primary key too much. `hash`
will always return `nil.hash` for records with no id, and `==` will
always return `false`. We can optimize those cases in the HABTM join
model.

Reduced memory leak problem in transactions by lazily updating AR objects with new transaction state. If AR object has a callback, the callback will be performed immediately (non-lazily) so the transaction still has to keep records with callbacks.

People need to make sure they are generated internally or added to attr_accessible so they can be mass assigned (for instance, from an import job).

Allows two models to use the same table but have different primary keys.

This reverts commit f6b50463.

Fear not, the roflscale will return when I have a bit more time and
figure out a better way to do it. (In particular, a way that doesn't
break the build.)

Don't prefix the name with attribute_. Avoids a string allocation on
read_attribute, which is a bit faster.

Also make it actually work.

It slows down all read_attribute accesses to map 'id' to whatever the PK
actually is, inside read_attribute. So instead make sure the necessary
methods are defined and that they redirect wherever they need to go.

This makes me happy!

This reverts commit ee2be435.

If we don't have a primary key when we ask for it, it's better to fail
fast. Fixes GH #2307.

Closes #2807.

This reverts commit c5448721.