I have a simple class hierarchy.

class Resource;
class Staff extends Resource;
class Coordinator extends Staff;

The corresponding tables are:

RESOURCE
STAFF
COORDINATOR

They join by primary key.

A Resource is a perspective Staff member, and a Staff member could become a Coordinator, but the subclasses all share the same information provided by the superclass, Resource.

My question is this.

Once I persist a Resource object, how do I change that resource object into a Staff or Coordinator object with Hibernate?

Working with Inheritance Objects is not documented extensively in the Hibernate Reference as of version 3.1.

Can you create the object as a Staff or Coordinator object and then persist it? I don't know how to change the class of an object after it has been created.

Your hbm.xml for Staff would start off with something like:

I have a method which copies all properties from one Resource to another.

Resource r = (Resource)Session.get(Resource.class, 1); // Get the Resource from Hibernate.
Staff s = new Staff(); // Create a new Staff object.
Resource.copy(r, s); // Copy the properties from the Resource to the Staff object.
Session.evict(r); // You must evict the Resource from the Session or you get "org.hibernate.HibernateException: Found shared references to a collection:" when you try to save, update or merge the resource.

-- To Persist the Change --
Session.merge(s); // Creates a new Resource & Staff record. I need it to update the existing Resource record and insert a new Staff record.
-- OR --
Session.update(s); // Produces a "org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect):"