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Hibernate: Merge vs SaveOrUpdate


Hibernate: Merge vs SaveOrUpdate

By seung | September 18, 2008
In my previous and current project, I have run into cases where I needed to save an object to database using hibernate, and from time to time, I run into some sort of Hibernate Session Exception.
 I did a bit of google search, and it turns out that when saving an object, I needed to make sure the object was attached to Hibernate session.
This can be very tedious issue, since that means if you need to save an object that is not attached to a session, you need to do these.
  1. Retrieve the object using the Id value found within the object passed to you.
  2. Update the property values one by one
  3. Save the object back within the same Hibernate Session
Here’s a quick summary that I found out about MERGE and SaveOrUpdate.
void saveOrUpdate(object)->object must be attached to a hibernate session (including all sub objects within the object), and once save/update is done, the object reflects the updated changes (e.g. primary key if saving a new object)

Object merge(object)-> object does not have to be attached to a hibernate session.  Once save/update is done, the object DOES NOT reflect the change.  The returned object reflects the changes, and it is attached to hibernate session.
MERGE method offers greater flexibility when it comes to saving data objects, since you need not worry about attaching object to Session.
You can create an object, set Id, and other properties on your own, and save it without worrying about the Hibernate Session exception.
If you’re using saveOrUpdate, the object saved MUST be attached to session.  I believe many people have experienced this issue.

This means you can create a copy of object from Service layer and just pass the object to your DAO.  Hibernate takes care of MERGING the data to appropriate hibernate session attached object and saves the data.
The only downside of using MERGE is that the object passed does not reflect the changed information.  So, if you need to use the updated object, you must get it from the returned object, not the parameter object.
I hope this helps. :)
-SK
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