java - JPA: Foreign key that is also a primary key mapping

Question

I have been trying to solve this for whole day but no luck! Also i tried to read most of the tutorials on the net but as you all know they all are full of useless examples that do not reflect what you need in the real world.

So here is my situation:

The database:

table: vehicles(vehicleId, brand, model, devYear, regNumber) <-- vehicleId is the PrimaryKey

table: extras(vehicleId, allowSmoke, allowFood, allowDrinks, airConditioner) <-- vehicleId is a PK and a FK.

The point is that if i have a class Vehicle and a class TravelExtras which are mapped to the database i want the Vehicle class to have an attribute TravelExtras travelExtras and get and set methods.

Unfortunatelly no matter what i tried when i try to persist the object in the databse i get various errors.

Here is an illustration:

        EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "NaStopPU" );
        EntityManager entitymanager = emfactory.createEntityManager( );
        entitymanager.getTransaction( ).begin( );


        TravelExtra travelExtra = new TravelExtra();


        entitymanager.persist(travelExtra);


        Vehicle vehicle = new Vehicle(2L, "10152487958556242", "Mazda", "626", "334343", 2005, 4);  
        vehicle.setTravelExtra(travelExtra);

        entitymanager.persist(vehicle);



        entitymanager.getTransaction().commit();
        entitymanager.close( );

        emfactory.close( );

Any one knows what kind of annotations to use for this One to one case ?

Answer 1

The Java Persistence wikibook has a section called Primary Keys through OneToOne and ManyToOne Relationships which seems to indicate that what you want is possible.

If I'm reading it right, for your case, it would look something like:

class Vehicle {
    @Id
    @Column(name = "EXTRAS_ID")
    private long extrasId;

    @OneToOne(mappedBy="vehicle", cascade=CascadeType.ALL)
    private TravelExtra extras;
}

class TravelExtras {
    @Id
    @Column(name = "VEHICLE_ID")
    private long vehicleId;

    @OneToOne
    @PrimaryKeyJoinColumn(name="VEHICLE_ID", referencedColumnName="EXTRAS_ID")
    private Vehicle vehicle;

    public TravelExtras(Vehicle vehicle) {
        this.vehicleId = vehicle.getId();
        this.vehicle = vehicle;
    }
}

Note that one of your entities will need to make sure it has the same id as the other, which is accomplished in the example by the TravelExtras constructor requiring the Vehicle it is bound to.