If you got two Entities A and B where A is a sample Entity with a 1:n relation to the Entity B
and B has a composite PK with the FK (A id) and a custom field like this:
If you try to persist an object of A holding one or more references to objects of B, the objects get correctly persistet. This means, that every inserted record of B has a correctly filled field "entity_a_id". The objects of B also have a correct reference to A holding now the new A record with its autoinc id.
But the object property "entity_a_id" stays empty. This leads to a collection of unusable EntityB objects (in memory), since on another persist of EntityA the values of entity_a_id are still not set correctly but the state is managed which leads to an "update B ... where entity_a_id=0"
We've tried to set the entity_a_id with an event handler on postPersist. On a persist of EntityA the collection entities wanted to update their entity_a_ids from 0 to the correct one.