Using ActiveRecord With (Previously) Derived Classes

If you have a class (in a pre-existing library, for example) that has
been derived from another class:

class ChildClass < ParentClass

since Ruby doesn't support multiple inheritance, you can't derive
ChildClass < ActiveRecord::Base to make ChildClass an ActiveRecord
class to persist ChildClass objects.

So what's the standard way to use ActiveRecord for
persistence/database operations on objects of already-derived classes,
such as ChildClass above?

I have to assume that the functionality of ChildClass and ParentClass
is irrelevant, we only want to implement CRUD database operations for
them as provided for with ActiveRecord. ParentClass and ChildClass
pre-exist in a library - you really shouldn't have to alter them to
avoid breaking client code that uses them. If the solution is to
create a mixin, what's the approach?