Hello, I a new user of hibernate

recently I had to start working with a mysql legacy database and one of things I have to do is to join two table though a pair o field which are not primary keys, or more specificly the filed in one of the table is not a primary keys, while the field of the second table is part of a composite key.

Unfortunatly modifing the database is not a option, could somebody give me an ideia of how to do this ?

Thanks in advaced.