Nhibernate entity with multiple Many-To-Many lists of the same type?

Question

Does anybody know how I would map an entity with two many-to-many collections of the same child type.

My database structure is this....

The "normal" relationship will be....

tbl_Parent
  col_Parent_ID

tbl_Parent_Child_Xref
   col_Parent_ID
   col_Child_ID

tbl_Child
   col_Child_ID

---

The alternative relationship is...

tbl_Parent
  col_Parent_ID

tbl_Include_ParentChild_Xref
   col_Parent_ID
   col_Child_ID

tbl_Child
   col_Child_ID

---

The entity and mapping look like this...

public partial class ParentEntity : AuditableDataEntity<ParentEntity>
{
  public virtual IList<ChildEntity> Children { get; set; }
  public virtual IList<ChildEntity> IncludedChildren { get; set; }
}

public partial class ParentMap : IAutoMappingOverride<ParentEntity>
{
    public void Override(AutoMapping<ParentEntity> mapping)
    {
     mapping.Table("tbl_Parent");

     mapping.HasManyToMany(x => x.Children)
     .Table("tbl_Parent_Child_Xref")
     .ParentKeyColumn("col_Parent_ID")
     .ChildKeyColumn("col_Child_ID")
     .Inverse()
     .Cascade.All();

     mapping.HasManyToMany(x => x.IncludedChildren)
     .Table("tbl_Include_ParentChild_Xref")
     .ParentKeyColumn("col_Parent_ID")
     .ChildKeyColumn("col_Child_ID")
     .Inverse()
     .Cascade.All();
    }
}

---

The error that I'm getting is "System.NotSupportedException: Can't figure out what the other side of the many-to-many property 'Children' should be."

I'm using NHibernate 2.1.2, FluentNhibernate 1.0.

Answer 1

You know I'm just shooting in the dark here, but it almost sounds like your ChildEntity class isn't known by Hibernate .. that's typically where I've seen that sort of message. Hibernate inspects your class and sees this referenced class (ChildEntity in this case) that id doesn't know about.

Maybe you've moved on and found the issue at this point, but thought I'd see anyway.

Answer 2

To my great pity, NHibernate cannot do that. Consider using another ORM.

Answer 3

Fluent is confused because you are referencing the same parent column twice. That is a no-no. And as far as I can tell from the activity i have seen, a fix is not coming any time soon.

You would have to write some custom extensions to get that working, if it is possible.

Answer 4

It seems FNH is confused because you seem to map the same object (ChildEntity) to two different tables, if I'm not mistaken.

If you don't really need the two lists to get separated, perhaps using a discriminating value for each of your lists would solve the problem. Your first ChildEntity list would bind to the discriminationg value A, and you sesond to the discriminating value B, for instance.

Otherwise, I would perhaps opt for a derived class of your ChildEntity, just not to have the same name of ChildEntity.

IList<ChildEntity> ChildEntities
IList<IncludedChildEntity> IncludedChildEntities

And both your objects classes would be identitical.

If you say it works with NH, then it might be a bug as already stated. However, you may mix both XML mappings and AutoMapping with FNH. So, if it does work in NH, this would perhaps be my preference. But think this workaround should do it.