This thread looks to be a little on the old side and therefore may no longer be relevant. Please see if there is a newer thread on the subject and ensure you're using the most recent build of any software if your question regards a particular product. Otherwise - post away!
We are using NHibernate Designer against a legacy database which is designed table per subclass as defined by the NHibernate reference manual: http://www.nhforge.org/doc/nh/en/index.html#inheritance-tablepersubclass
AFAIK NHibernate Designer only supporst table per hierarchy inheritance.
Any possibility of adding support for table per subclass in the near future?
We'll have beta support for table per subclass in the next nightly build, available from about 1200 GMT. You'll see a new "Inheritance Type" setting on the inheritance arrow; if you change this to Table Per Subclass, a new "Key" setting appears for you to specify the subclass join key column.
We would welcome feedback if this meets your requirements, and of course do let us know if you run into any bugs!