No persister for... {SUBCLASS} NHibernate with Fluent NHibernate

How do I let NHibernate ignore extra properties of a subclass of my model?

class SuperModel { // hot I know
{
    public Guid Id { get; private set; }
    public string FirstName { get; set; }
}

class SubModel : SuperModel {
    public string FavoriteColor { get; set; }
}

I really only want to store the SuperModel data using my repository and use the FavoriteColor elsewhere, but I get

No persister for: SubModel

even though I save it with my repository as

void Store(SuperModel model) {
   using (var session = Session){
       session.SaveOrUpdate(model); // <<<< The exception is thrown here
   }
}

and some where else I use

void WhatToDo(SubModel model) {
   doSomething(model.FavoriteColor);
}

And I use it as such

var model = new SubModel { FirstName = "Miranda", FavoriteColor = "Green" };
modelRepository.Store(model);
someService.WhatToDo(model);

Any one know how I can fluently configure this? Thanks.

FYI- implicit and explicit casting has no effect.

Edit

My mappings are like this

class SuperModelMap : ClassMap<SuperModel>
{
    public SuperModelMap()
    {
        WithTable("SuperModels");
        Id(x => x.Id);
        Map(x => x.FirstName);
    }
}

Edit 2

I figure/found out that I could do this, but in my database, I have to have a dummy table, which would just be inefficient. It works but there has to be a better way...

In my SuperModelMap...

JoinedSubClass<SubModel>("SubModel", MapSubModel);

private void MapSubModel(JoinedSubClassPart<SubModel> part)
{
    // Leave this empty
}

Edit 3 I'm closer, but I still get a different error on selection.

I tried this.

DiscriminateSubClassesOnColumn("Id")
    .SubClass<SubModel>(m => { });

InnerException {"Object with id: 5586b075-47f1-49c8-871c-9c4d013f7220 was not of the specified subclass: SuperUser (Discriminator was: '1000')"} System.Exception {NHibernate.WrongClassException}

NHibernate assumes that you'd like to retrieve exactly the same object as you persist. So, even though you don't care about additional properties, you might care about the type of object. If you don't, the simplest solution would be to make a shallow copy of SubModel object, but instead of creating SubModel object, create SuperModel object.

I assume you thought about this and didn't like it. If you'd like to avoid dummy table, but can live with dummy column, I'd suggest you call:

DiscriminateSubClassesOnColumn("dummycolumn")
.SubClass<SubModel>(m => { });

This column would be used by NHibernate to store information about persisted object's type. When you load object from the db, it will be SubModel or SuperModel, depending on what it was when you persisted it.

Your solution with calling DiscriminateSubClassesOnColumn didn't work, because NHibernate couldn't determine which class to use based on id column.

Another idea: I'm not sure if it will work, but you could add another mapping, for SubModel, exactly the same as for SuperModel. Then, NHibernate should persist SubModel to the same table as SuperModel, and when you request your object it should fetch SuperModel object. Unfortunately I can't test this solution right now, maybe you can get it to work. No SubClass in this solution - two "parallel" mappings.