Questions to the Session.merge() function

racs · **Joined:** Mon Aug 22, 2005 10:50 am **Posts:** 1

Hibernate version: 2.1

We are currently designing a web-based application which shall use Hibernate for ORM

In our App we create Instances of Objects (let's say a Student named Patrik with several attributes) and pass the Instance to a DTO-Factory which creates a XML-File representing the Student. (Session closed, Instance is detached or even destroyed).

The XML-File is sent to a Client where it will be modified. The Client then sends back a XML-File containing (in the worst case) only an Identifier and one attribute (e.g. the phone Number) which has been changed.

The XML-File ist then processed by the DTO-Factory which instanciates an Instance of Student that has only the Identifier and Phone Number set to proper values (the Rest is asumed to be NULL).

Now, what happens exactly, when the merge() function is called with the (crippeled) Student Instance. (New Session)

Is the function intelligent enough to copy only the phone Number into the Database, or will the rest of the attributes of the Student be set to NULL, or will happen some other strange effects?

The main question behind this scenario is:
Do we need to keep the Instance of the Student in a (e.g. http-) Session and implement our on Version of a merge() which applies the changes made in the XML-File to this instance (and than use SaveOrUpdate() ), or is Hibernate able to handle this by the build-in merge()-function?

Any help appreciated!

rajeshwar_rao · **Posted:** Mon Aug 22, 2005 11:23 am

I am not sure weather this is what you are looking for. But when your changes comes back from the client. You may just load the Student befor the update. then set the changed values and I belive changes would be saved accordingly.

Frankly I never tried this but logically it looks like it should work this way.

But then y does the client not want to send the entire XML back with changes and rather decides to send only one parameter value??

ramnath · **Posted:** Tue Aug 23, 2005 12:30 am

Hi

I have used merge() it sets NULL to other values in the table.

SO i have used the HQL update for specific column. If you know that the column is specific you can use HQL. Look in to the below link for an example.
http://www.hibernate.org/hib_docs/v3/re ... tch-direct

If it varies on the runtime then you can load the Object with save() and then use update() or saveorupdate() to update the value.