In the end I think the pain of implementing this seamlessly was not worth the gain provided. The intention was that it would allow plain ruby objects that might not live in your main application to be subclassed and have persistence mixed in. But I've decided that the benefit of doing that is not worth the amount of complexity that the implementation introduced.
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.
This reflects the fact that it now impact inserts as well as updates.
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.
Previously we would just silently write the attribute. This can lead to subtle bugs (for example, see the change in AutosaveAssociation where a through association would wrongly gain an attribute. Also, ensuring that we never gain any new attributes after initialization will allow me to reduce our dependence on method_missing.
…d there (and this call to super is no essential, so easiest to simply remove)