Selecting objects not in a collection in NHibernate with ICriteria Interface

In my system Users own 0 or more Categories. Here is a simplified version of my model classes:

public class User
{
    public virtual String Name {get; set;}
    pub开发者_如何学编程lic virtual IList<Category> Categories { get; set; }
}

public class Category
{
    public virtual String Title {get; set;}
}

I now want to create an ICriteria query to select all categories that aren't assigned to a user, but I'm stuck. Ideally I don't want to create a navigation property from Category to User, but with my beginner knowledge of NHibernate that's the only solution I can see.

Is there an ICriteria query that will do this with the current data model classes?

Thanks for your help.

This is off the top of my head, but might be a useful pointer.

var crit = _session.CreateCriteria<Category>("c")
                      .Add(
                        Subqueries.PropertyNotIn("c.id",
                            DetachedCriteria.For<User>("u")
                                .CreateCriteria("Categories","uc")
                                .SetProjection(Projections.Property("uc.id"))                                    
                        ));
var unassignedCategories = crit.List<Category>();

You can probably get a feel for the SQL that will be generated here:

select c.* from categories where c.id not in (select uc.id from usercategories)

Hope this helps, and sorry I haven't been able to test it :)

Tobin