!images/polymorphic.png(Polymorphic Association Diagram)!
+Polymorphic associations can get tricky with inheritance. If you need
+an inheritance relationship in your ActiveRecord models, you should use
+Single Table Inheritance instead.
h4. Self Joins
In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations: