Well, what TeaMoe's approach does is look for an existing model and use it for the association. This should be ok for most cases but the problem arises when the model we are testing is the only existing model, pretty odd though. To be honest, I'm not happy at all with my solution, but after struggling with it a couple of hours this is the simpler way I've found.

Another approach could be to use the TeaMoe's commit but raise an exception with a nice message stating that another model must be defined. Any thoughts?