How can you replicate Hibernate's saveOrUpdate in JPA?

Question

In JPA, is there any way you can replicate Hibernate's saveOrUpdate behavior,

saveOrUpdate

public void saveOrUpdate(Object object)
                  throws HibernateException

    Either save(Object) or update(Object) the given instance, depending upon resolution of the unsaved-value checks (see the manual for discussion of unsaved-value checking).

    This operation cascades to associated instances if the association is mapped with cascade="save-update".

    Parameters:
        object - a transient or detached instance containing new or updated state 
    Throws:
        HibernateException
    See Also:
        save(Object), update(Object)

which essentially checks to see if the object already exists in the database and either updates that object as need be or saves a new instance of the object.

JPA transcationless reads are nice, but I am really missing this method from Hibernate. How do experienced JPA developers handle this?

Answer 1

Try using the EntityManager.merge method - this is very similar.

There is an excellent description of the differences in Xebia's blogpost: "JPA Implementation Patterns: Saving (Detached) Entities."

Answer 2

The problem with the method outlined in the article that Pablojim linked to, is that it doesn't handle auto generated primary keys very well.

Consider the creation of a new ORM entity object, you can give this the same data as an existing row in the database table, but unless I am mistaken, the entity manager does not recognize them as the same row until they have the same primary key, which in a entity that uses auto generated keys, you can't get until you go up to the database.

Here is my current work around for that situation;

/**
 * Save an object into the database if it does not exist, else return
 * object that exists in the database.
 *
 * @param query query to find object in the database, should only return
 * one object.
 * @param entity Object to save or update.
 * @return Object in the database, whither it was prior or not.
 */
private Object saveOrUpdate(Query query, Object entity) {
    final int NO_RESULT = 0;
    final int RESULT = 1;

    //should return a list of ONE result, 
    // since the query should be finding unique objects
    List results = query.getResultList();
    switch (results.size()) {
        case NO_RESULT:
            em.persist(entity);
            return entity;
        case RESULT:
            return results.get(0);
        default:
            throw new NonUniqueResultException("Unexpected query results, " +
                    results.size());
    }
}