I think it's going to be too much pain to try to transition the
:active_record load hook from executing against Base to executing
against Model.

For example, after Model is included in Base, and modules included in
Model will no longer get added to the ancestors of Base.

So plugins which wish to be compatible with both Model and Base should
use the :active_record_model load hook which executes *before* Base gets
loaded.

In general, ActiveRecord::Model is an advanced feature at the moment and
probably most people will continue to inherit from ActiveRecord::Base
for the time being.

* Use each_key instead of generating intermediate keys array.
* Use each_with_object instead of inject to build hash.
* Use ternary to return instead of if + assignment.

Rather than doing it every time an instance is instantiated.

Get rid of ActiveModel::Configuration, make better use of
ActiveSupport::Concern + class_attribute, etc.

When subclassing abstract_class table_name should be always computed
based on class name, no matter if superclass is subclassing base
or another abstract_class. So:

class FirstAbstract < ActiveRecord::Base
  self.abstract_class = true
end
class SecondAbstract < FirstAbstract
  self.abstract_class = true
end

class Post < SecondAbstract
  self.table_name #=> 'posts' (not 'second_abstracts')
end

This is the 'top level' connection, inherited by any models that include
ActiveRecord::Model or inherit from ActiveRecord::Base.

The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.

So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.

The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.

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