I tend to agree that this is a bug. I also spent nearly a day trying to work out what was going on. I am using JPA2.0 annotation/objects with Hibernate 4.1.4. My example is as follows:

• Employee entity with a ManyToOne relationship with a Schedule Entity. * The Employee entity also has a OneToMany relationship with a Qualification Entity.
• The Schedule entity has an ElementCollection (java type List) of Embeddable type ScheduleEntry (e.g. start/stop time).
• The ElementCollection is set for Eager loading.

Let's assume I have an employee record with four qualification entries and a Schedule with a single ScheduleEntry record. When I load the Employee object via EntityManager.findById, Hibernate will issue a single SQL SELECT joining the results, thus retrieving four rows. Since the ScheduleEntry table has no unique identifier, hibernate will populate the List in the Schedule object with 4 copies of the ScheduleEntry.

I fail to see any situation where this is desirable and in all situations it seems to be the wrong result. Changing between lazy and eager fetching should not result in a user being delivered an object with entirely different content, and in the latter case, the wrong content.

I also don't see why it is required. All of the information is available for hibernate to detect this situation, and when it exists, use two SQL queries. As far as I am aware, EAGER doesn't require a single SQL request be used, just that all fields should be populated during the find call.

I understand that using a Set will solve this, but that seems like a hack at the user level. And I can see situations where multiple copies of identical objects might be required.

EDIT:
I am using H2 v1.3.167, not MySQL