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?

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>();

Check if this works (I have only tested it with QueryOver on NH 3, but it seems to be the same): stackoverflow.com/a/19717548/806975 — Guillermo Gutiérrez
– Guillermo Gutiérrez, Commented Oct 31, 2013 at 22:19

Felice Pollano · Accepted Answer · 2011-01-17 14:41:01Z

15

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

answered Jan 17, 2011 at 14:41

Felice Pollano

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10 Comments

UpTheCreek Over a year ago

Thanks. I'll take a look at that. Do you know why this is not the default behaviour? Who would want the duplicates?!

Felice Pollano Over a year ago

This is the same question I asked when I faced your problem. Not a real answer, but let's think what happen in the query side: the resulting recordset will contain duplicates, and NH mimic this background behavior. In any case NH internally uses the same entity, they are only duplicated entries into the returned list.

UpTheCreek Over a year ago

@Felice Pollano, just added the resultstransformer. Unfortunately this did not change anything for me - I'm still getting the duplicates. Perhaps this is down to the type of collection?

Mauricio Scheffer Over a year ago

DistinctRootEntity only applies to root entities, which is not the problem here.

Robot Mess Over a year ago

@FrancoisBotha: It also works for me as long as I only have one join, the moment I have 3 joins it stops working.

|

Mauricio Scheffer · Accepted Answer · 2011-01-18 01:50:50Z

11

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.

edited Jan 18, 2011 at 1:50

answered Jan 17, 2011 at 15:34

Mauricio Scheffer

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3 Comments

schmmd Over a year ago

What if you were fetching 1000 entries instead of a unique result? Surely two JOINs (Cartesian product) in a single SQL query is more efficient than 2001 selects, even if it returns more rows (this depends in part on how large the collections are).

schmmd Over a year ago

I may have jumped the gun--you are right in most cases. It probably makes sense to issue two queries, one with a single join.

Johan Haest Over a year ago

I used the Future methode to fix this, NH did the rest for me.

schmmd · Accepted Answer · 2011-02-11 06:21:35Z

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...

answered Feb 11, 2011 at 6:21

schmmd

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Comments

JoshBerke · Accepted Answer · 2011-03-18 15:50:53Z

3

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.

answered Mar 18, 2011 at 15:50

JoshBerke

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Comments

Guillermo Gutiérrez · Accepted Answer · 2013-10-31 22:17:41Z

2

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;

answered Oct 31, 2013 at 22:17

Guillermo Gutiérrez

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