02 June 2010

A note about Cascade.MERGE

Al salamo Alykom,

Hi folks, I've a long time since I've been posted about Java EE.
So, today we gonna talking about JPA, we are talking about the merge operation.

From Specs:

The semantics of the merge operation applied to an entity X are as follows:
....
- If X is a managed entity, it is ignored by the merge operation, however, the merge operation is cascaded to entities referenced by relationships from X if these relationships have been annotated with the cascade element value cascade=MERGE or cascade=ALL annotation.


This is what I need to talk with you about:

Here' an example:
Consider we have a uni-directional many-to-one relationship between Jobs and the user that do that job:


public class User{
// .. user data here...
}

public class Job{

private User user;
//....

public User getUser(){
return user;
}
}



as it appears, It is a uni-directional from Job to User (many-to-one)

suppose we have the following annotation over the getUser operation:
@ManyToOne(cascade=CascadeType.MERGE)

this means, when a managed Job have changed its User, on merging the Job Object to DB, also merge the in-relation Object (User).


Here's a code example that prove this specs element:

Job job = em.find(Job.class, 1);
User u = job.getUser();
u.setName("mohammed");
em.merge(job);


After this code, the field name of User will be updated to be "mohammed".
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