title: Map many to many relationship when join table has its own PK
body: <p>I have to map a relationship Person with Tasks. OK.
  My problem is that I cannot alter the DB and the JOIN table 'Person_Task' has this fields: <code>PERSON_TASK_ID, PERSON_ID, TASK_ID</code>
  with a sequence for <code>PERSON_TASK_ID</code>.</p>
  
  <p>When I insert a new Task I get a beatiful:
  <code>
  java.sql.SQLIntegrityConstraintViolationException: ORA-01400: cannot insert NULL into ("USER"."PERSON_TASK"."PERSON_TASK_ID")
  </code></p>
  
  <p>It would be really nice if someone can help me to accomplish this <strong>without creating a new entity</strong> :)</p>