Hi,

I've read both the Hibernate and EJB3 documentation on CascadeType.MERGE.

I understand the concept of cascading a merge() operation but I am not clear on what exactly this means in the following case:

class Dog
{
/**
* Each dog goes after a specific bone. Many dogs may go after the same bone
*/
@ManyToOne(cascade={CascadeType.CREATE, CascadeType.MERGE})
Bone getBone();
}

Now say in the above case, if we set a Bone associated with Dog and persist the Dog, then the Bone object will get persisted too right (because of CascadeType.CREATE)? I understand that...

Now when is CascadeType.MERGE actually used? If the Bone associated with a dog changes, does MERGE imply that the original Bone is removed from the DB? Or maybe does it imply that if a Dog's Bone is modified, then that change is persisted to the DB? Or does it imply anything else?

Please clarify when CascadeType.MERGE should be used through some small use-case. (It would be nice if the Hibernate documentation included an example too)

Thank you,
Gili

I think I better clarify *exactly* what I'm trying to do...

Image
\-> ManyToOne -> ImageSpecification

ImageSpecification
\-> height
\-> width


So basically many images might have the same image specification and if I create a new image with an image specification that doesn't already exist, I want it created. Fine so far. Now, if you modify the ImageSpecification associated with the Image it shouldn't modify the underlying ImageSpecification table. The original entry should be left alone (other Images might be using it) and a new ImageSpecification should be pointed to (created if necessary).

The final requirement I have is: ImageSpecifications should be removed (automatically?) if no Image refers to them. Sorta like garbage collection.

Now, I'm not sure that you could do all of this in Hibernate but I would appreciate your advice on it. My original thoughts is that ImageSpecification should be declared with:

@ManyToOne(cascade={CascadeType.CREATE})

Notice there is no cascade on MERGE. And then I guess I'd have to manually scan the ImageSpecification table on a nightly basis and delete any unused entries. Is this the best I can do?

Thanks,
Gili

if bone is modified and dog is merged



then the bone representation in DB will be updated thanks to the merge cascading.

Don't cascade MERGE to the image specification


Will not remove bone since it's a many to one

Hmm, in the case of a many-to-one relationship, will it *ever* remove bone? I mean, is Hibernate smart enough to remove the bone if no other dog refers to it or is it up for me to do manually?

Thank you,
Gili

No, you have to do that manually.