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Hibernate version:
1.0.2 and 1.2.0.Beta1
I have spend a lot time to trying to figure this out and it is important to get this resolved before I can recommend Nhibernate on our project so Thx much for your help.

I have two concrete sub-classess that implement an Interface.

Both the sub-classess have identical attributes, however they have different associations.

One of the concrete subclass let's say ClassA has a many-to-many association with another class, let's say ClassB.

Therefore ClassB has a property, which is a collection of ClassA. After I reconstitute ClassB from the database and access its property (collection of ClassA objects); What I get back is a collection of nhibernate proxies that can only be cast to the super class Interface.

Nhibernate throws an exception when I try to cast an proxy to ClassA. I need to cast it to ClassA, because ClassA has associations which is not shared by the Interface or other sub-class ClassB.

The exception message I get is -
Unable to cast object of type 'ProxyInterfaceSystemObject_IPerson_INHibernateProxy_ISerializable' to type 'NHibernateQuickStartExample.Professor'.

BTW I have run into this problem while implementing Justin Gehtland's tutorial - http://www.theserverside.net/tt/article ... NHibernate

For those of you who have implemented this tutorial, restating the problem in terms of the objects in the tutorial - I retrieve "Department" from the database. I access its property that represents a collection of "Professors". However, I can only cast each object in the Professors collection as a "Person" which is an Interface.

I communicated this issue with Justin, he thinks maybe the libraries may have changed since he wrote the article in 2004.

The quick and dirty solution:

Maybe I'm missing something, but the other solution seems to be to declare Department.Professors as a collection of Professors, not as a collection of Person as it is now.

Finally, I'd say the domain model is broken. What happens if a Professor decides to take a class? How does the model in the example allow someone to be both a Student and a Professor? It doesn't. Of course, it can be forgiven because it's just an example. But for a real system, you would want Professor, Staff, Student, etc. to be a role, not a class.

Inheritence is not the answer to every modelling problem.

rponton,

I thought I may have to explicitly retrieve each individual Professor as you did in the first response. I take it when NHibernate retrieves the Department.Professors it returns a collection of proxies and the proxies can only be cast to an Interface.

But your second response suggests I could directly get back Professors as a collection of Professor's or maybe I am not following, Can you please elaborate your comment - "the other solution seems to be to declare Department.Professors as a collection of Professors, not as a collection of Person as it is now".

Isn't it true in NHibernate you can only declare collections as one of the collection interfaces such as bag, set, etc so I cannot possible declare Department.Professors as a collection of Professors.

btw Your first response works fine, Thx much for your help.