setting an id autogenerate into an object

Question

Sorry if my post is duplicated or the tittle doesn't describe the topics, because I don't know how to describe this in the tittle, I look on internet, but I didn't find the solution.

I am using Java and JPA. The problem is the next :

I have a class A with an autogenerated key :

class A{
      @Id
      @GeneratedValue(strategy=GenerationType.IDENTITY)
      private int id;
      private List<B> listB;
}

And the class B with the id of this clas:

 class B {
         @EmbeddedId
         private Bid id;
         private String att;
     }
     class Bid {
         private int idA;
         private String text;
     }

In a controller I want to create an object A, the problem is when I created the object A, I need to create the object B where the id of B contains the id of A which is autogenerated, and it is created in the moment when the entity is mapped to de database, I dont't know how to set the id autogenerated of A into the idB, maybe I should query to de database asking what is the las id of classA, but it seem bad.

Thanks in advance

Answer 1

Your case is a derived identifier case, where your entity B's identity was derived from the primary key of A. You can use @MapsId annotation for this case and your entities can be restructured like this:

@Entity
public class A {

  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private int id;

  @OneToMany(mappedBy="a")
  private List<B> listB = new ArrayList<B>();
  ...
}

@Entity
public class B {
    @EmbeddedId
    private BId id;

    @ManyToOne
    @MapsId("idA")
    private A a;
    ...
}

@Embeddable
public class BId {
    private int idA;
    private String att;
    ...
}

This is how you would persist the entities:

A a = new A();

BId bid = new BId();
bid.setAtt("text"); // notice that the idA attribute is never manually set, since it is derived from A

B b = new B();
b.setId(bid);
b.setA(a);

a.getListB().add(b);

em.persist(a);
em.persist(b);

See sample implementation here.