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()
                .SetFetchMode(\"ChildColl         


        

Answer 1

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 2

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 3

Try using Futures:

ICriteria criteriaFuture1
    = session.CreateCriteria<MyRootType>()
        .SetFetchMode("ChildCollection1", FetchMode.Eager)
        .Add(Restrictions.IdEq(id))
        .SetResultTransformer(Transformers.DistinctRootEntity)
        .FutureValue<MyRootType>();
 ICriteria criteriaFuture2
    = session.CreateCriteria<MyRootType>()
        .SetFetchMode("ChildCollection2", FetchMode.Eager)
        .Add(Restrictions.IdEq(id))
        .SetResultTransformer(Transformers.DistinctRootEntity)
        .FutureValue<MyRootType>();

 return criteriaFuture1.Value;

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.

Answer 5

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