nHibernate inheritance problem when saving childs

Question

Hello. I'm having a problem: I have User:

[Class(Table = "Users", Name = "User")]
public class User
{
    [Id(0, Column = "UserId", Type = "Guid", Name = "Id")]
    [Generator(1, Class = "assigned")]
    public Guid Id { get; set; }

    [Property(Name = "Password", Column = "Password", Type = "String")]
    public string Password { get; set; }

    [Property(Name = "Email", Column = "Email", Type = "String")]
    public string Email { get; set; }
}

And a ditributor:

 [JoinedSubclass(ExtendsType = typeof(User), Table = "Distributors")]
public class Distributor : User
{

    [Id(0, Column = "DistributorId", Type = "Guid", Name = "Identifier")]
    [Generator(1, Class = "assigned")]
    protected Guid Identifier { get; set; }

    [Property(Name = "Company", Column = "Company", Type = "String")]
    public String Company { get; set; }
}

Now I need to promote user to distributor. But I need to save his id (other parts of the system are using it). I'm getting user and creating new distributor. But when I'm trying to save distributor I'm getting this error:

a different object with the same identifier value was already associated with the session: ec6f6a9f-a236-4385-835c-7f408a5f594d, of entity: MLMCore.Entities.Distributor

I have tryed all the methods (save, SaveOrUpdade, SaveOrUpdateCopy, Update). But it doesn't works. Any ideas how to fix it?

Answer 1

The error is what it says - your session contains an instance of User with that Id, so when you try to save another User (a Distributor) with that Id, NHibernate complains. You could try clearing your session (session.Clear()) before saving the Distributor, but I don't know if NHibernate will be successful in saving to the User and the Distributor table suddenly.

If you are in a position to change your design, I suggest you do that - generally, you shouldn't use inheritance when the type of your specialization could change. It's much better and more convenient to introduce some kind of Role in your domain.

Perhaps I can give you some inspiration, even though I don't know the specifics of your domain: How about letting users be just that - instances of User. And then, User might container an IList<Role> and you could specialize Role into e.g. Distributor (and other roles if that's relevant)?

Or how about letting your company contain an IList<Distributor>, each pointing to to a User?