Let's imagine a situation.

[HierarchyRoot]
    public abstract class AbstractObject : Entity
    {
        [Field, Key]
        public Guid Id { get; private set; }
        ...
    }

    public abstract class BlogPost : AbstractObject 
    {
        [Field]
        public BlogCategory Category { get; set; }
        ...
    }

    public sealed class BlogCategory : AbstractObject
    {
        ...
    }

I want to get an object with some ID.

var post = Query<BlogPost>.SingleOrDefault(id);

If this ID is Category's ID, I get an exception instead of returning NULL. I think it is not a right behavior.

Updated at 28.10.2009 14:58:16

[InvalidCastException: Unable to cast object of type 'W3P.Model.BlogCategory' to type 'W3P.Model.BlogPost'.] Xtensive.Storage.Query`1.SingleOrDefault(Object[] keyValues) in c:\TeamCity\buildAgent\work\6a5d81309423096e\Xtensive.Storage\Xtensive.Storage\Query{T}.cs:87

...

Updated at 28.10.2009 15:09:12

Yes. I suppose a category object was fetched earlier and stored in cache.

This thread was imported from our support forum. The original discussion may contain more detailed answer. Original topic by buck.