Basically, what it amounts to is; the foreign key fields have to match the primary key fields in number and type.
My scenario involved two classes w/ 3 PK fields each...but the relationship between the two involved only two of the fields. This is impossible in JPA today and makes it very hard to use it w/ very poorly designed legacy data.
[Message sent by forum member 'zambizzi' (zambizzi)]
http://forums.java.net/jive/thread.jspa?messageID=215428