(This may already exist and I just don't know about it, but I don't think it's in the docs...)
If a relationship has a compound key, right now you can get it if you do something like this, where a Rating maps to a registration by childID and seasonID:
This is really cool.
The only drawback is that it requires that the whole object is loaded before the relationship can be got, which means you can't eager load a relationship this way. I can't remember how exactly it fails - whether it just uses the seasonID from the first entity or just doesn't construct the query correctly – but it makes sense that it fails since eager loading works off the relationship attributes and the relationship doesn't know that there is more than one key.