I am struggling with Hibernate 1-to-many mapping(collection).
Could someone tell me how to manage the collection of one-to-many mapping? a little code will be very helpful.

I have a very simple one-to-many relationship: an Item has many Picture(s).

ENTITY -- Item
@...
public class item {
private Collection pictures;
private Integer id;

// getter/setter for id and @Id(...)


@OneToMany(cascade = {CascadeType.MERGE}, fetch = FetchType.EAGER, mappedBy="item")
public Collection getPictures() {
return pictures;
}

public void setPictures(Collection pictures) {
this.pictures = pictures;
}

public void addPicture(Picture p) {

if (pictures == null)
pictures = new ArrayList();
pictures.add(p);

}
}

ENTITY Picture --
@ ....
public class Picture {
private Item item;
private Integer id;

// constructor
public Picture(Item item, ....) {
this.item = item;
....
}
// getter/setter for id and @Id(...)

@ManyToOne
@JoinColumn(name="itemId")
public Item getItem() {
return item;
}

public void setItem(Item item) {
this.item = item;
}
}

I create an Item in persistence without any pictures. Then use a session bean to add pictures to the item.

Session Bean --

@Stateless
public class ItemManagerBean implements ItemManager {

public void addPictureToItem(Item item, Picture p) {
item.addPicture(p);
em.merge(item);
}

}

The merge() ignores those pictures that are already in persistence.
Add picture P1: ---> persistence: P1, ( great!)
Add picture P2 ---> persistence: P1, P1, P2 (P1 duplicated)
Add picture P3 ---> persistence: P1, P1, P2, P1, P2, P3

Obviously, the merge() ignored what are already in persistence. All pictures has Id. EM.merge() should be able to know P1 is already in persistence when adding P2.

I f I do not call merge(), nothing will be persisted.

I do this from Web GUI. Transaction is thread-based.(default)

Do I miss anything? or Is this a Hibernate bug ?


-----------------

I tried one thing: Create a picture in persistence using another session bean. Then use em.refresh(item). First time it works, and I can see the picture from item.getPictures().
Then I create another picture the same way, and call em.refresh(item) again, but this time item.getPictures() does not have the second picture.
Not that I have changed the Cascade type to REFRESH.

Basically what is the way to manage the collection in one-to-many mapping?

I am using JBoss 4.0.3RC1 and HSQL db on XP.

Please help! Thank you.

Dave

After merge, I found the id of Picture is null. I expect the picture id will be populated from persistence after merge. So this might be the reason Hibernate ignore what in the persistence. How to solve this problem? Is this a bug? Thanks.

Read the merge() JavaDoc before using it.

Hi emmanuel,
I read javadoc. I still have not figure out how to manage collection for one-to-many mapping. I appreciate it if you can throw me some light.
Suppose A has Many B(s). I am still struggling. When persisting/merging A, B are created, but B does not have Id. Thanks.!

Work on the graph that is returned by the merge operation, not the one you've injected

HI emmanuel,
I do not understand. I new to Hibernate. Could you explain? Thanks.

a' = em.merge(a);
work on a', not a