Hibernate version:
Hibernate 2.1.4
Mapping documents:
Reference.hbm.xml
Code between sessionFactory.openSession() and session.close():

Full stack trace of any exception that occurs:

Name and version of the database you are using:
Microsoft SQL 2000

I'll describe exactly what I mean with a use case:

In our system we have directory (Directory), sites (SiteBean_a), and units.

Directories have sites, sites have units.

We would like to extend Directory, sites (SiteBean_a), and units (UniteBean) from a base class