JPA How can I get the generated id/object when using merge from parent but child is created?

Question

I have an entity that has been previously persited and has a @OneToMany relationship with another entity. In order to add a new entity I just add my new entity in the managed object and use cascadeType.ALL to persist the changes. Is there a way that I can get the id's of the newly created objects or get the original (unmanaged) object I used with the merge to have its id updated?

In pseudo-code I would expect the following to happen:

1. New copy is going to be returned for merged entity
2. Old copy is going to be update for new entities

Example: Parent A, id=13 Child B, id=0

In essence, I want to issue a merge on the parent but cascade persist on child (so that the original child instance is updated, not copied).

Obviously this doesn't happen. I am using hibernate as the provider.

Answer 1

Stackoverflow post and JPA documentation have the answer provided that you do your research.

The way to do what I want is to use persist on the managed parent. This will ignore any changes on the parent, but will cascade persist (provided that it is set up to cascade). The child object will have the correct id afterwards.

....
JPAEntity newObject=new JPAEntity();
managedObject.addChild(newObject);
em.persist(managedObject)
newObject.getId() //work fine!

Answer 2

You should be able to "see" a generated ID for a new Entity:

• after transaction commits, or

• after a em.flush() (where em is your EntityManager) while a transaction is active.

Note also that all relationships between Entities need to be resolved in the Java data structures prior persistence. Child references to parents need to be "set", and vice-versa.

Answer 3

I had a similar problem with persistence provider eclipselink. I needed to add new orders from a client database with temporary "client orderIDs" to an existing customer in a server database. Because I needed the "server orderIDs" for further client requests, i wanted to create a map (clientID, serverID) and send it back to the client. Basically my approach is to get a reference to the copy of the newly added child order. It will always be added to the end of my list in parent class customer, that fact is used to retrieve it.

I use the entity classes Customer and Order, where Customer has a List (LinkedList) of orders.

Parent Customer class: ...

    @OneToMany(mappedBy = "customer", cascade=CascadeType.ALL, orphanRemoval = true)
    private List<Order> orders = new LinkedList<Order>();

    public Order getLastOrder() {
      return orders.get(orders.size()-1);
    }

Child Order class:

    @ManyToOne(optional=false)
    private Customer customer;

Servlet:

    Customer customer = customerService.findByID(customerID); //existing customer
    Order order = new Order();
    //add attributes to new order
    customer.addOrder(order);
    customerService.update(customer); //generates keys for children
    order = customer.getLastOrder();
    Long orderID = order.getID; //Nullpointer exception

CustomerService updates customer with EntityManager.merge but I forgot to update the reference in the servlet:

CustomerService class:

    public void update(Customer entity) {
      entityManager.merge(entity);
    }  

I solved it this way: CustomerService class:

    public Customer update(Customer entity) {
      return entityManager.merge(entity);
    }

Servlet: ....

    customer = customerService.update(customer); //generates keys for children

Now everything works fine.