Strange behaviour on fetch = FetchType.EAGER in @OneToMany

erikaxel · **Joined:** Sat Aug 26, 2006 3:56 pm **Posts:** 1

Hibernate version:
Hibernate Annotations 3.2.0.CR1
Hibernate 3.2 cr2
Name and version of the database you are using:
MySQL 5.0.24-standard

Hi,

When I'm using @OneToMany(fetch = FetchType.EAGER) the rows in my parent table are doubling when i fetch them.

To specify more clearly I have made an example using almost the same code as in the documentation with Troop and Soldier. The code I use can be found at http://folk.ntnu.no/erikaxel/hibernate/ There I also have the listing of the tables I am using.

The point of interest is in Troop.java:

Code:

@OneToMany(mappedBy = "troop", cascade = {CascadeType.REMOVE}, fetch = FetchType.EAGER)
 public List<Soldier> getSoldiers() {
     return soldiers;
}

When I try to list all Troops with:

Code:

Session session = sessionFactory.openSession();
Transaction tx;
tx = session.beginTransaction();
List<Troop>list = session.createCriteria(Troop.class).list();
for(Troop troop : list)
   System.out.println("Troop id:" + troop.getId());
tx.commit();

I get the following output
Troop id:1
Troop id:2
Troop id:2
Troop id:3
when I am using fetch = FetchType.EAGER as specified above. If I remove that but keep everything else I get
Troop id:1
Troop id:2
Troop id:3
as expected.

In the tables troop 1 has 1 soldier, troop 2 has 2 soldiers and troop 3 has 0 soldiers.

I have tried to search the forums and the JIRA but haven't found anything. To me this looks like a bug, but I thought I would post it here first in case I'm wrong. If this is wanted behaviour, please let me know.

-- Erik Axel

melix · **Joined:** Thu Aug 31, 2006 4:05 am **Posts:** 5

Hi,

I'm seeing this too. It's a serious problem for us as I can't simply replace the EAGER with a LAZY fetch strategy (though it would be better, agreed).

I don't know if it's a Hibernate Core problem (fixed in CR4 ?) or an annotations one (as annotations are not compatible with CR4 yet, can't say).

assaf27 · **Joined:** Sun Jan 22, 2006 6:56 am **Posts:** 29

I think with criteria queries with joins you need to add
c.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY) to avoid the duplicate results.

melix · **Joined:** Thu Aug 31, 2006 4:05 am **Posts:** 5

Well, I'm quite new to Hibernate, but the main reason why I can't just replace EAGER with LAZY is that I use a generic class for DAOs. The problem with it is that the session is lost, so calling lately getXXXX() on collections throws a session not found error.

BTW, how could I tune this code in order to get the fix working ?

Code:

public abstract class ADaoPrototype<TBean extends AHibernateBean> extends HibernateDaoSupport implements IDao<TBean> {

   private Class<TBean> thePersistentClass;

   private Class<TBean> getBeanClass() {
      if (thePersistentClass != null) return thePersistentClass;
      thePersistentClass = (Class<TBean>) ((ParameterizedType) getClass()
            .getGenericSuperclass()).getActualTypeArguments()[0];
      return thePersistentClass;
   }

   public TBean findById(Serializable id) {
      return (TBean) this.getHibernateTemplate().get(this.getBeanClass(), id);
   }

   public void save(TBean aBean) {
      this.getHibernateTemplate().saveOrUpdate(aBean);
   }

   public void update(TBean aBean) {
      this.getHibernateTemplate().update(aBean);
   }

   public List<TBean> getAll() {
      return this.getHibernateTemplate().loadAll(this.getBeanClass());
   }

   public void remove(TBean aBean) {
      this.getHibernateTemplate().delete(this.getHibernateTemplate().load(this.getBeanClass(), aBean.getSerializableId()));
   }

   public void closeSession() {
      this.getSessionFactory().close();
   }
}

[/code]

assaf27 · **Joined:** Sun Jan 22, 2006 6:56 am **Posts:** 29

melix, do you have the same problem (duplicates entries in query result) as the thread starter or are you hijacking the thread? ;)

Anyway if you have the same problem, then the only method that could possibly cause this problem is the getAll method (because it's the only one that returns a list)

So I suggest you try to replace

Code:

return this.getHibernateTemplate().loadAll(this.getBeanClass());

With:

Code:

DetachedCriteria c = DetachedCriteria.forClass(this.getBeanClass());
c.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY);
return this.getHibernateTemplate().findByCriteria(c);

HTH

assaf27 · **Joined:** Sun Jan 22, 2006 6:56 am **Posts:** 29

BTW, using eager fetching on collections as a rule is rarely the solution (unless you have a small data model or don't care about performance). Lazy is on by default for a good reason.
There are other solutions for avoiding all those LazyInitializationExceptions. For example of a pretty sophisticated technique, google for "Open Session In View".

melix · **Joined:** Thu Aug 31, 2006 4:05 am **Posts:** 5

I do have the same problem : replacing LAZY with EAGER fixes the problem ;) I'll try your fix and try to see if it works fine for me.

Tx.

emmanuel · **Posted:** Tue Sep 05, 2006 3:28 pm

Just to be clear, this is not a problem but a behavior (the correct approach has been described before).
And get rid of hibernateTemplate, use the plain APIs