I can't work out why values from the beans aren't being bound properly. This is my code:

Session session = HibernateUtil.currentSession();
Transaction tx = session.beginTransaction();

InterviewQuestion iq = (InterviewQuestion)f.get("generic_1"); //from UI

session.lock(iq, LockMode.READ);
Iterator iter = iq.getAnswerOptions().iterator();

while(iter.hasNext()) {
AnswerOption ao = (AnswerOption)iter.next();
if(ao.isSelected()) {
iq.addInterviewAnswer(new InterviewAnswer(iq, ao)); // not binding
}
}

session.saveOrUpdate(iq); // cascade="save-update" in config
tx.commit();
HibernateUtil.closeSession();

This is the (cut down) log that shows nulls being bound:

Hibernate: insert into Interview_Answers (INTERVIEW_QUESTION_ID, ANSWER_ID, ID) values (?, ?, ?)
net.sf.hibernate.type.LongType - binding null to parameter: 1
net.sf.hibernate.type.LongType - binding null to parameter: 2
net.sf.hibernate.type.LongType - binding '2' to parameter: 3

Thanks!

Have you read http://www.hibernate.org/hib_docs/reference/html/parent-child.html ?