It seems that feature (table per concrete class) was added to Java Hibernate 3. Unfortunately NHibernate was ported from Hibernate 2 and there is no <union-subclass> element.

But I still hope experimenced developers will know any trick how to map the following scenerio:

I have base abstract class Employee and tow subclasses: DefaultEmployee and ExternalEmployee, each with different bussiness behavior. There are two tables in my database, where some columns (personal number, name, title and email) are exactly the same, other columns differ - this is suited to <union-subclass>. It might be better to redesign the database to suit the <joined-subclass> mapping strategy. But i can't make any changes to this legacy database because it would break another applications.

Is there any workaround how to make this "table per concrete class mapping" in NHibernate?

<union-subclass> was added in Hibernate 3 and isn't implemented in NH; however Hibernate 2 supports table per concrete class using implicit polymorphism (see
http://www.hibernate.org/hib_docs/reference/en/html/inheritance.html#inheritance-strategies). I haven't use this in NH, but I can't see any reason why it shouldn't work.

if the keys of the tables are different you can create a table Person with only the id and use de <joined-subclass> mapping strategy.
With this way you don