Hi everyone,
I am facing a problem that I was not able to solve on my own, thus I ask for your help. I have an entity class which is almost a POJO, containing several properties which should be considered by the persistence provider but I also have another property in the entity class which is a collection and which is annotated as transient.

As soon as I try to merge such a detached entity, the collection - which was non-empty before the merge - becomes empty right after the merge. This is a behaviour that I do not understand and which causes unwanted side effects, because I need to store the entity before processing any further, because I need its ID in order to save the entities that are stored in the collection.

Just in case that you suggest to let the JPA do the dirty work - this is not possible here because some of the entities need transformation prior to persisting.

Could anyone explain this behaviour?

Thank a lot,
Florian

merge creates a copy of the persistent data and return a copy, persist makes the object managed.
You might want to check saveOrUpdate (from the Hibernate Session).

I'm having the same problem and I use @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE}, fetch=FetchType.EAGER)

In short. Parent class has a collection of Child class.

I invoke EntityManager.merge with a newly created Parent object with associated Child object that are already persistent.

During the execution of EntityManager.merge Hibernate invokes the Parent.setChildren method with an empty Collection