Question

I have two entitys, A and B. A has a one to one relationship of B.

    class A {
        String aValue;
        B b;
    }
    class B {
       String bValue;
    }

The class B stuff are already pre populated. Now, a user on a website selects what B he wants to submit. On the server we get an Id of what B that was.

The Question:
How do I create a new A, without actually do a DB query asking for the B that has this id. I mean, the A table in database only has a id reference. One should be able to set that Id without fetching the B.

Answer 1

If you are using JPA and Assuming the following:

@Entity
@Table(name="A")
class A {
    @Id
    @Column(name="id")
    private int id;
    String aValue;
    @OneToOne
    B b;
    // Getters, setters and other stuff
}

@Entity
@Table(name="B")
class B {
   @Id
   @Column(name="id")
   private int id;
   Integer Id;
   String bValue;
   // Getters, setters and other stuff
}

If you now the id of entity B. You can simply make persist as follows:

B b = new B();
b.setId(1000); //Assuming that you know the id. 
               //There is no necessary fill all the object. Just the PK is needed.
A a = new A();
a.setId(100);
a.setAValue("nothing");
a.setB(b);
em.persist(a);

I have tested this with Hibernate as Persistence Provider. If you are using other ORM please specify it.