Transient object Exception

rfkrocktk · **Joined:** Mon Jul 07, 2008 8:07 pm **Posts:** 42

I'm still learning the do's and don'ts about Hibernate and I had a question. It has to do with persisting collections and cascading. Here's my pseudo-code:

Code:

Main Entity Class: 
...
   @OneToMany (
      mappedBy = "mainEntity",
      targetEntity = Things.class,
      fetch = FetchType.EAGER,
      cascade = CascadeType.ALL
   )
   private List<Things> things= new ArrayList<Things>();
...

Collection Entity Class:
...
@ManyToOne @JoinColumn(name = "main_entity_id")
   private MainEntity mainEntity;

...

Now, when I go to update values in the things collection inside the MainEntity class, nothing happens. My code to add values to the List is this:

Code:

mainEntity.getThings().add(value);
mainEntity.getThings().addAll(values);

When I do this, nothing happens as far as Hibernate is concerned. Yes, the Things objects are added to the transient MainEntity class, but not to the database in any way. So, I have to call this:

Code:

mainEntity.getThings().addAll(values);
List<Things> thingList = new ArrayList<Things>();
for (Things thing : mainEntity.getThings()) {
    thing.setMainEntity(mainEntity);
    thingList.add(thing);
}
mainEntity.setThings(thingList);

myDao.update(mainEntity);

whenever I make an update to the collection values. Kind of a bit of a workaround, wouldn't you say?

Now, if I omit this code and just call update after adding values to the mainEntity.getThings() List, I get a Transient Object Exception (HibernateException) saying that the added Things don't belong to the mainEntity instance or something like that.

To prevent redundancy and wasting of time, is there a way I can do this without having to define all of this for every collection I persist?

gonzao_diaz · **Joined:** Mon Nov 26, 2007 2:29 pm **Posts:** 443

rfkrocktk ,

Whenever you use a child collection from a parent object, bear in mind that you are using a "leaky abstraction". This means that Hibernate will keep the fiction that "whatever you add to the child collection will end up in the database", but only only to a certain extent.
As soon as you start creating separate lists and using addAll(), you basically "lost" Hibernate, it stops working for you.

In your example, you are defining a bidirectional association. In those cases, one of then does the real SQL work, and the other has to be marked "inverse". How is that reflected in your code?

Also, consider that the proper way to assign a child, in your parent class, is to create your own custom method, which not only adds the child to the list, but sets itself as the parent of this new child.

Code:

public void addThing(Thing obj){
  this.thingsList.add(obj);
  obj.setParent(this);
}

Of course, you can easily modify this to add several children at a time.
But, do you understand why a simple addAll() won't do?

rfkrocktk · **Joined:** Mon Jul 07, 2008 8:07 pm **Posts:** 42

Gonzalo,
Thanks, it totally makes sense.

I've actually been attempting to create a unidirectional association between two entities, but I've been totally unsuccessful in doing that up to this point. Without including the reference in the owned entity, I have been unable to map a collection using a join column or join table.

I'll definitely implement what you've suggested into my code, thanks so much for your help.
- TK

tkassembled · **Joined:** Fri Nov 21, 2008 3:35 pm **Posts:** 12

If you haven't already, view the Hibernate documentation online. It's worth reading through to understand the ins and outs of Hibernate.