When making a composite key referring to a target entity something like the following ORM XML should be defined:
<?xml version="1.0" encoding="UTF-8"?>
<many-to-one name="owner" id="true">
But for some reason the id attribute is not picked up by Hibernate, and therefore it does not interpret the relation as a primary key. A workaround is to add a id tag referring to the same property, but it would be nice of Hibernate would pick up the id attribute on the many-to-one relation.