Apparently we have no mapping solution for composite key cases as described at:
https://forum.hibernate.org/viewtopic.php?f=9&t=1024512

I think that's a major use case and we should definitely provide a solution.
I'm pretty sure that I did such mappings in the past and don't remember what kind of workaround I might have used - that was a long time ago and it might not be possible anymore - or if it is, we should document the strategy.