Weird issue with merge using JPA...?

Question

Hi all,
I have the following code snippet (HsTrans has a manytoone relation with Hss), Hss is the parent
            HsTrans hsTrans = new HsTrans();
         Hss hss=em.find(Hss.class, studyId);
         hsTrans.setStudy(studyId);
         hsTrans.setState(state.toString());
         hsTrans.setTimestamp(timeOfState);
         hss.getHsTrans().add(hsTrans); //Add child to aprent
         hsTrans.setHss(hss);                //Child Parent
            em.merge(hss);  // Does not work Throws a Unique Key violation error
Whereas replacing the same with this
         Hss hss2=hsStudyTrans.getHsStudy(); 
            em.merge(hss2); // Works fine
   Does anyone know if this is a JPA bug or some other issue.
Thx
VR

Answer 1

RainaV wrote:
Does anyone know if this is a JPA bug or some other issue.JPA is a specification, so it has no bugs (only design flaws). If there is a bug somewhere it is in the persistence provider you are using, but most likely it is just you not understanding how merge and transaction management in general works. I've written dozens of applications now that use JPA and I think I really needed merge a grand total of once.
In this snippet you provide, you are apparently making the 'hss' object managed through the em.find() method. This means that any changes you make to it will be made persistent as soon as the transaction is committed. You don't need to call merge() at all, you only call that on entities that are detached.
Now for the actual problem of unique key violation, I don't even see you committing hsTrans. Change the code to this:
HsTrans hsTrans = new HsTrans();
Hss hss=em.find(Hss.class, studyId);
hsTrans.setStudy(studyId);
hsTrans.setState(state.toString());
hsTrans.setTimestamp(timeOfState);
hsTrans.setHss(hss); // set managed hss object
em.persist(hsTrans); // persist hsTrans and make it managed
hss.getHsTrans().add(hsTrans); // add the new managed hsTrans to the hss mapped collection