Now that's a bit trickier since the user has provided the metadata for the key column name.

What exactly do you mean by this? I'm not sure how to find out which column is the map key, i.e. I don't know how to find out that "nick" should be used as the key.

BTW all this work must also be done on all document supported backend (ie CouchDB).

Yes, my first PR for this issue unifies the association handling for MongoDB and CouchDB to a certain extend and thus applies this change to both backends.