Hi,

I have a class hierarchy as follows.

Abstact Class Object1 (name, date)
Class Object2 inherits Object1 (age)
Class Object3 inherits Object1 (IList Links)

Ive employed a one table per subclass mapping for the above. Object2 adds extra columns which are mapped as a joined subclass, this is fine and dandy. However Object3 doesn't add any extra columns but instead has a collection property of links from a Links table.

What is the best way to map this? Creating a table for Object3 seems pointless as they share the same Identifier as the Object1 table.

What I want is a joined subclass without a table but this sounds wrong.

Any help greatly appreciated.

You can't mix subclass mapping strategies for a given class hierarchy, so either you need to have a table that has nothing other than a primary key or you need to use another mapping strategy.

An alternative is to use the table-per-class-hierarchy strategy where you have one table for both concrete subclasses. You add a discriminator column so NHibernate knows which rows represent which type and make any columns that are not present in the base class nullable.

There really isn't a best practice for this - it depends on your specific situation.

Cheers,

Symon.

ok thanks for that. I would like to keep the table per subclass at the moment as I think its cleaner. Ill create the empty table for the time being. Is there a problem with having a joined-subclass with no property mappings say, if I want a concrete instance of the abstract class with no extra properties or collections?

Nope, no problem.

It might be a little less performant for your database if it's under heavy load as it needs to do the extra table lookup required by the join but with a clustered index on your primary key I don't imagine it's likely to hurt you unless you're in a high performance situation.

Symon.