Hi Magnus Hovén, the Id needs to be mapped to a single field.
You are correct that you need to apply a custom TwoWayFieldBridge, but your bridge needs to be able to somehow encode (and decode) the various parts making up the composite key into a single term.

I didn't expect this to work at all, I'll create a follow up issue to see if we can apply better validation of the mapping. Thanks for the detailed explanation!