I didn't define the function - it's from DataMapper. And it's designed to work with normal fields. It's designed specifically to mirror the functionality of a compound primary key.
The reason field validation rules won't work against relationships is that the relationships aren't set until after the object has been saved. You might be able to make a related validation rule work, but I don't usually do much with those.
The reason I think this absolutely belongs in a controller is this is a business logic function. This is something that needs to be enforced through application logic. I don't understand why it would ever be possible for the user to pick an invalid combination in this instance. In general, it's more a matter of opinion, and design choice.
