Why do the NH docs say that "subclass" (one table per class hierarchy) is recommended over "joined-subclass" (one table per class)?

I think subclass is ugly because you end up with columns in your database which may or may not be used depending on the value of another column. This prevents you from using constraints to enforce data integrity.

Also, is it possible to use bags, many-to-one, etc., etc. with a joined-subclass? (i.e., only the subclass has these elements). The docs seem to indicate that it is not possible.

I don't know about your first quastion, but about the seconde you ar wrong.
I do can use bag, many-to-one ... into the joined-subclass.
take a look at the xsd or dtd, it is realy good.

The name already says it: a joined subclass always uses a join when retrieving an object. Now this isn't too bad when you have a single subclass, but especially polymorphic queries with many different subclasses can result into lots of joins that might degrade performance.
As always: this is no golden rule. You have to test it for yourself if this is an issue or not.
Also, I might have forgotten something here, but there are some Java Hibernate docs that explain this stuff.



It is perfectly possible to do that.