problem with mapping table in NHibernate

Question

Hey, I rewrite application from ASP.NET to Silverlight and use NHibernate 2. Tables in my db looks like this:

In Dziecko table I have reference to id of Opiekun and Grupy. In ASP it's ok for me but in videos Summer with NHibernate I see that Stephen Bohlen in exchange use object of this type, for me its Opiekun and Grupy.But in SQL Server I can't define object type, so better is map only this id[will I could problem querying for id] and or something else ?? Using all object of specific type is efficiency? We must query all object in exchange only int type of id's ??