I'm working with a child table that doesn't need to load the full parent, it only needs to know the id. If the child object is assigned a valid foreign key, it works fine. The issue is that the foreign key property can be null in the datasource. How do I let NHibernate know that a zero value is saved as dbnull? Or is there a better way to map it then using a simple <property> tag?

I think you will have to use a NullableInt32 type from NHibernateContrib.