I really don't like this solution, but I added a field on the model: @attr() anotherModelSelected; and in the function that is setting anotherModel also set anotherModelSelected to true (and false when cleared). Then in the schema definition, ignored anotherModel and tracked anotherModelSelected instead.

I don't like it because it adds another field to the model when the actual field we WANT to test is already present in the model, but I'll roll with this for now...