Error: self-reference "must be nullable as it represents a hierarchical relationship" exception

 
I've got an exception that has been occurring for some time (Sept 2010?), and it still exists in the latest nightly build. I've been getting around it by reverting portions of my generated model code manually to a version that previously worked (more on that later).

Basically the problem lays in a table that has a reference to itself.  Let's call it "Foo" -- given a (Guid) key of FooId, there is a nullable field called ParentFooId that has a foreign key to FooId, although the db foreign key has no cascade action.

I've set up a One-to-Many-Association to represent this relationship, with properties set to their defaults except IsNullable and IsDependent is set to true (RemovalBehavior remains set to 'SetNull', although this has no effect).

Here's the exception that occurs:

Mindscape.LightSpeed.LightSpeedException: The field ParentFooId on type [Foo]
 must be nullable as it represents a hierachical relation
   at Mindscape.LightSpeed.LightSpeedException.?(Exception , String , Object[] )

   at Mindscape.LightSpeed.LightSpeedException.?(String , Object[] )
   at Mindscape.LightSpeed.Model.ToManyModel.?()
   at Mindscape.LightSpeed.Model.AssociationModel.?(TypeModel )
   at Mindscape.LightSpeed.Model.TypeModel.?(IEnumerable`1 )
   at Mindscape.LightSpeed.Model.TypeModel.GetTypeModel(Type )
   at ♣.☼.?(Query , IList )
   at Mindscape.LightSpeed.UnitOfWork.Find(Query query, IList results)
   at Mindscape.LightSpeed.UnitOfWorkBase.Find(Query query)
   at Mindscape.LightSpeed.Linq.Plan.SingleQueryPlan.ExecuteImmediate

(IUnitOfWork unitOfWork, Type returnType)
   at Mindscape.LightSpeed.Linq.LinqQueryProvider.Execute(Expression expression)

   at Mindscape.LightSpeed.Linq.LinqQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
   at System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source)
[...]

I'm running postgresql 9, and have confirmed that ParentFooId is in fact nullable, both in the db and the model.  I've done a diff against the previously generated model code that does work versus the one that doesn't.  Basically the difference relates to the placement of [DependentAttribute]:

In the Working version this exists:

    [DependentAttribute(ValidatePresence=false)]
    [ReverseAssociation("ParentFoo")]
    private readonly EntityCollection<Foo> _foos = new EntityCollection<Foo>();

But in the 'new' non-working version this has been removed, and instead the designer places the attributes as follows:

    [DependentAttribute(ValidatePresence=false)]
    private System.Guid _fooId;

    [Column("ParentFooId")]
    [DependentAttribute(ValidatePresence=false)]
    private System.Nullable<System.Guid> _parentFooId;

I did try to set the Is Dependent property on the association to false (in order to manually delete child records), but that resulted in a stack overflow when attempting to save a graph of parent/child objects!

Any ideas? Is this a bug or something I'm doing?

UPDATE: with the last nightly build, I've discovered I'm now getting a stack overflow doing an update of this table.  Due to application logic I:

a) create the object (and save it as an insert) with ParentFoo to null
b) do some processing, including building a graph of any child objects
c) update the ParentFooId to the ID of the parent, or if there is no parent, I set it to ID?

Could 'c' be the root of both problems, in that I'm doing a self-reference in the event that there is no parent?  Does Lightspeed require that hierarchical relationships eventually terminate in a null, and not a self-reference?

Thanks!

An update: I did manage to figure out a work-around.  I removed the association (the ParentFoo object property) and now I just manually work with the field directly (ParentFooId).

However, for future reference, is there a design issue with Lightspeed where hierarchical relationships must never have parentid == childid to reflect an item with no parent (and instead forces parentid == null)?

Thanks! 

That is correct.  An item with parent id = child id would mean an item which is its own parent.  To indicate an item with no parent, use a null parent id.  (Which is why associations to self are required to be nullable, otherwise the parent hierarchy could never bottom out.)

The additional occurrence of DependentAttribute is correct and is the result of a designer fix.