Error: "Cannot simultaneously fetch multiple bags" when calling Configuration.BuildSessionFactory();

Question

We are getting this error after upgrading to NHibernate 2.1.

[QueryException: Cannot simultaneously fetch multiple bags.]
   NHibernate.Loader.BasicLoader.PostInstantiate() +418
   NHibernate.Loader.Entity.EntityLoader..ctor(IOuterJoinLoadable persister, String[] uniqueKey, IType uniqueKeyType, Int32 batchSize, LockMode lockMode, ISessionFactoryImplementor factory, IDictionary`2 enabledFilters) +123
   NHibernate.Loader.Entity.BatchingEntityLoader.CreateBatchingEntityLoader(IOuterJoinLoadable persister, Int32 maxBatchSize, LockMode lockMode, ISessionFactoryImplementor factory, IDictionary`2 enabledFilters) +263
   NHibernate.Persister.Entity.AbstractEntityPersister.CreateEntityLoader(LockMode lockMode, IDictionary`2 enabledFilters) +26
   NHibernate.Persister.Entity.AbstractEntityPersister.CreateLoaders() +57
   NHibernate.Persister.Entity.AbstractEntityPersister.PostInstantiate() +1244
   NHibernate.Persister.Entity.SingleTableEntityPersister.PostInstantiate() +18
   NHibernate.Impl.SessionFactoryImpl..ctor(Configuration cfg, IMapping mapping, Settings settings, EventListeners listeners) +3261
   NHibernate.Cfg.Configuration.BuildSessionFactory() +87

Without stepping into the NHibernate source, it doesn't look like I can see which mapping is creating the issue.

It's a very old application with a load of mappings files, lots of mappings have one-to-many bags in them, all lazy instantiated.

For example:

    <bag name="Ownership" lazy="true" cascade="all" inverse="true" outer-join="auto" where="fkOwnershipStatusID!=6">
        <key column="fkStakeHolderID"/>
        <one-to-many class="StakeholderLib.Ownership,StakeholderLib" />
    </bag>

maps to:

public virtual IList Ownership {
        get {
            if (ownership == null)
                ownership = new ArrayList();
            return ownership;
        }
        set { ownership = value; }
    }    

Has anyone seen this error before when upgrading to NHibernate 2.1?

Answer 1

We found the answer eventually...

We had an entity that had more than 1 bag with a many to many relationship that had

outer-join="true"

set.

This caused the error because when you set outer-join fetching you usually do this to limit the db round-trips, enabling nHibernate to retrieve the whole association in 1 select. In this circumstance this will cause a pre-fetch on each bag immediately causing the error.

We changed that to outer-join="auto" (as below) and they stopped being pre-fetched. This stopped nHibernate attempting to fetch multiple bags simultaneously.

<bag name="Groups" lazy="true" cascade="none" table="dbname..tablename">
    <key column="foreignkeyname" />
        <many-to-many class="classname.typename,assemblyname"
                column="foreignkeyname" outer-join="auto" />
</bag>