Is it possible to have some sort of discriminator field for a composite-element just like a subclass? The reason being is that I would like to define a class heirachy just using composite-element and not having to resort to subclasses.

Subclassing components isn't supported in NHibernate yet. You'll need to do an entity association to accomplish that.

MIKE

Can you provide an example of an entity association?

All an entity association means is that the entity that is in the collection has its own 'class' mapping file, as opposed to being a component within another class mapping.