Question

I’m converting an Eclipselink application to use Hibernate 3.6 bundled with JBoss 6 instead, and need to modify how lazy loading is done because of this. It is not my first time using hibernate.

However, it seems that my collections become uninitialized after a merge is done. I cannot remember seeing this behaviour ever before. For example:

Entity entity = entityDAO.getEntity(id);
System.out.println(entity.getMyCollection().size()); // OK, no exception
entity = entityDAO.update(entity);
System.out.println(entity.getMyCollection().size()); // Throws LazyInitializationException 

EntityDAO snippet:

public Entity getEntity(Long id){
    Entity e = entityManager.find(Entity.class, id);
    Hibernate.initialize(e.getMyCollection());
    return e;
}

public Entity update(Entity entity){
    return entityManager.merge(entity);
}

Entity snippet:

@OneToMany(mappedBy="entity", cascade=CascadeType.ALL, orphanRemoval=true)
private List<AnotherEntity> myCollection = new ArrayList<AnotherEntity>();

Is this really the expected behaviour? It seems very weird that hibernate would throw away data during merge..

Answer 1

That’s indeed expected behavior. merge() consists in

• getting the entity with the same ID as the detached one passed as argument,
• copying the state of the detached entity to the attached one,
• and returning the attached entity

So, obviously, if you don’t reinitialize the lazy collection of the returned attached entity, the collection won’t be loaded.