Active Record uses a base class which other classes inherit via subclassing to become “activated”. This presents some problems for pre-existent object models, namely those that already have parent classes. For these, a mixin is needed instead of a class.
On its own, this wish is of limited benefit, but in conjuction with the Direct Object Relational Mapping Wish?, becomes extremely useful.
Keep in mind that both the base class and the mixin module can be supported simultaneously, maintaining backward compatability.
With something like EvilRuby you can have Multiple Inheritance which might help here.