NHibernate returning duplicate object in child collections when using Fetch

Question

When doing a query like this (using Nhibernate 2.1.2):

 ICriteria criteria = session.CreateCriteria<MyRootType>()
                .SetFetchMode("ChildCollection1", FetchMode.Eager)
                .SetFetchMode("ChildCollection2", FetchMode.Eager)
                .Add(Restrictions.IdEq(id));

I am getting multiple duplicate objects in some cartesian fashion. E.g. if ChildCollection1 has 3 elements, and ChildColection2 has 2 elements then I get results with each element in ChildColection1 one duplicated, and each element in ChildColection2 triplicated! This was a bit of a WTF moment for me...

So how to do this correctly?

• Is using SetFetchMode like this only supported when specifying one collection?
• Am I just using it wrong (I've seen some references to results transformers, but imagined this would be simplier).
• Is this something that's different in NH3?

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Update:

As per Felice's suggestion, I tried using the DistinctRootEntity transformer, but this is still returning duplicates. Code:

ICriteria criteria = session.CreateCriteria<MyRootType>()
                .SetFetchMode("ChildCollection1", FetchMode.Eager)
                .SetFetchMode("ChildCollection2", FetchMode.Eager)
                .Add(Restrictions.IdEq(id));

 criteria.SetResultTransformer(Transformers.DistinctRootEntity);

 return criteria.UniqueResult<MyRootType>();

Answer 1

You can select single distinct results by specifying SetResultsTransformer(Transformers.DistinctRootEntity);

Answer 2

You're doing a cartesian product here. Don't. Instead, fetch each collection separately.

BTW: this isn't something NHibernate-specific, the same applies to any ORM in any platform, or even pure SQL without any ORM at all. In general, you don't want to fetch N*M rows when you can fetch N+M instead.

Answer 3

I am not using NHibernate (rather, regular 'ol Hibernate), but in the Java version of Hibernate you can define One-To-Many collections as either Lists or Sets. If you define them as Sets you will not get duplicates. Surprisingly enough, this happens without overriding equals(). Unfortunately, I have the same problem and I want Lists so I can integrate with Wicket but no duplicates...

Answer 4

What were you mapping your collection as? If your using a bag for example you will get duplicates. You can use a set instead and you won't get duplicates.