When bulk inserting entities, you might get an error similar to this:

Exception in thread "main" java.lang.ClassCastException: org.hibernate.action.internal.DelayedPostInsertIdentifier cannot be cast to java.lang.Long
at java.lang.Long.compareTo(Long.java:54)
at org.hibernate.internal.util.compare.ComparableComparator.compare(ComparableComparator.java:41)
at org.hibernate.internal.util.compare.ComparableComparator.compare(ComparableComparator.java:36)
at org.hibernate.type.AbstractStandardBasicType.compare(AbstractStandardBasicType.java:225)
at org.hibernate.action.internal.CollectionAction.compareTo(CollectionAction.java:172)
at org.hibernate.engine.spi.ExecutableList.add(ExecutableList.java:222)
at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:250)
at org.hibernate.event.internal.AbstractFlushingEventListener.flushCollections(AbstractFlushingEventListener.java:274)
at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:103)
at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:55)
at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1222)

This is better illustrated in the attached sample application.

This is the main logics causing problems:

for (int i = 0; i < 10000; i++) {
    final Entity1 entity1 = new Entity1();
    session.persist(entity1);

    final Entity2 entity2 = new Entity2();
    session.persist(entity2);

    // session.flush();

    final Criteria criteria = session.createCriteria(Entity2.class);
    entity1.setEntities(criteria.list());

    if (i % BULK_SIZE == 0) {
        session.flush();
     }
}

If you uncomment the first flush operation, it will work, but at a highly reduced performance level.

If you comment out the collection setter, you'll see that the application finishes successfully in a few seconds.