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

Answer 1

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 orders = new LinkedList();

    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.