Question

I am wondering what the best practice would be for updating a record using JPA? I currently have devised my own pattern, but I suspect it is by no means the best practice. What I do is essentially look to see if the record is in the db, if I don’t find it, I call the enityManager.persist(object<T>) method. if it does exist I call the entityManager.Merge(Object<T>) method.

The reason that I ask, is that I found out that the the merge method looks to see if the record is in the database allready, and if it is not in the db, then it proceeds to add it, if it is, it makes the changes necessary. Also, do you need to nestle the merge call in getTransaction().begin() and getTransaction.commit()? Here is what I have so far…

   try{
        launchRet = emf.find(QuickLaunch.class, launch.getQuickLaunchId());
        if(launchRet!=null){
            launchRet = emf.merge(launch);
        }
        else{
            emf.getTransaction().begin();
            emf.persist(launch);
            emf.getTransaction().commit();
        }
    }

Answer 1

If the entity you’re trying to save already has an ID, then it must exist in the database. If it doesn’t exist, you probably don’t want to blindly recreate it, because it means that someone else has deleted the entity, and updating it doesn’t make much sense.

The merge() method persists an entity that is not persistent yet (doesn’t have an ID or version), and updates the entity if it is persistent. You thus don’t need to do anything other than calling merge() (and returning the value returned by this call to merge()).

A transaction is a functional atomic unit of work. It should be demarcated at a higher level (in the service layer). For example, transfering money from an account to another needs both account updates to be done in the same transaction, to make sure both changes either succeed or fail. Removing money from one account and failing to add it to the other would be a major bug.