Exceptional NonUniqueObjectExceptions

libardo · **Joined:** Thu Jan 04, 2007 3:56 pm **Posts:** 11

I'm scratching my head with an exception thrown intermittedly when saving objects with NHibernate 2.0-2.0.1.

All in all it's very similary to this: http://forum.hibernate.org/viewtopic.php?p=2396478

I'm posting again not to hijack the thread. Perhaps this contains further clues.

The context is
- NH 2.0 or 2.0.1
- ASP.NET

What I'm thinking at this point is that the exception only occurs when:
- using MySQL
- saving "copies" of objects (I'll post more details further down)
- cascading inserts of mapped entities
- (SQL server and SQLite seem to work fine)

So far my debugging spree has led me to this place in the NH codebase:

Code:

   AbstractSaveEventListener::PerformSaveOrReplicate
    - source.ActionQueue.Execute(insert);

It appears my entity goes in with id 0 and comes out with an identity already used by another entity. When I venture further down the inserts and identity selects looks okay as far as I can tell.

This is an exceprt of my HBM:

Code:

...
    <map name="Details" inverse="true" cascade="all-delete-orphan" generic="true" where="DetailCollectionID IS NULL" lazy="true">
      <cache usage="read-write" />
      <key column="ItemID" />
      <index column="Name" type="String" />
      <one-to-many class="N2.Details.ContentDetail, N2"/>
    </map>
...
  <class name="N2.Details.ContentDetail,N2" table="n2Detail">
    <cache usage="read-write" />

    <id name="ID" column="ID" type="Int32" unsaved-value="0">
      <generator class="native"/>
    </id>
    <discriminator column="Type" type="String" />
...

This is my Equals and GetHashCode:

Code:

           public override bool Equals( object obj )
      {
         if( this == obj ) return true;
         ContentDetail other = obj as ContentDetail;
         return other != null && id == other.id;
      }
      public override int GetHashCode()
      {
         if (id > 0)
            return id.GetHashCode();
         return base.GetHashCode();
      }

What about that copying? I've previously used memberwiseclone and cleared specific fields (such as the details collection). I've also tried to create a new object using activator and assigning specific fields to no effect.

Other oddities include a not completly deterministic execution. I wouldn't swear by it but it seems timing related.

I'll probably continue banging my head against this for a while but I'd gladly accept hints of any kind. Thank you for your time.

wolli · **Posted:** Fri Dec 19, 2008 6:04 am

You may run into problems with that:

Code:

public override int GetHashCode()
      {
         if (id > 0)
            return id.GetHashCode();
         return base.GetHashCode();
      }

The hash code changes after the object gets an id, which happens during save. Try to use a constant hash code.

libardo · **Joined:** Thu Jan 04, 2007 3:56 pm **Posts:** 11

I tried removing the GetHashCode override (which should revert to reference based calculation) but it didn't help.

Is it a bad idea to use an incremental ID fto calculate the hash code?

wolli · **Posted:** Sat Dec 20, 2008 9:04 am

Have look here:

http://nhforge.org/blogs/nhibernate/archive/2008/09/06/identity-field-equality-and-hash-code.aspx