After having slept on it, I'm not sure how to proceed on this. I'd rather not commit to a common base class, since I am not sure that will work long term and would be annoying to roll back*.

Maybe we move can the `MTLSelectorWithKeyPattern`-part into a `MTLModel` category to at least slim it down a bit?

* unless there is an elegant way to have a common base class without exposing it, that'd probably be too clever though.