To get all the ActiveRecord goodies your class must inherit from ActiveRecord::Base:
class Post < ActiveRecord::Base
Why not ask the user to include ActiveRecord as a module instead?
The advantage of this approach is that Rails could automatically include ActiveRecord in all classes in the models directory, making it completely transparent to the user. I.e.:
class Post # no `< ActiveRecord::Base`!
Is this a (relatively minor) design flaw in Rails?
ActiveRecord is not Hibernate. ActiveRecord models are not pure models - they are explicit Active Records and follow the semantics of the Active Record pattern.
This is not a design flaw. This is a completely different design goal.
I would certainly like to see a Ruby Hibernate. But ActiveRecord is not it.
Martin Fowler describes an Active Record as:
An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data.