I was reading the NH Doc. and found this:
Quote:
4.4. Modular mapping files
It is possible to define subclass and joined-subclass mappings in seperate mapping documents, directly beneath hibernate-mapping. This allows you to extend a class hierachy just by adding a new mapping file. You must specify an extends attribute in the subclass mapping, naming a previously mapped superclass. If you are configuring NHibernate by using Embedded Resources then the hbm.xml files are configured in the correct order. If you are manually adding them or specifying them in the cfg.xml file then the ordering of the mapping documents is important!
I hope this help you.
Bye