JPA - Persisting a One to Many relationship

Question

Hi,

Maybe this is a stupid question but it's bugging me.

I have a bi-directional one to many relationship of Employee to Vehicles. When I persist an Employee in the database for the first time (i.e. it has no assigned ID) I also want its associated Vehicles to be persisted.

This works fine for me at the moment, except that my saved Vehicle entity is not getting the associated Employee mapped automatically, and in the database the employee_id foreign key column in the Vehicle table is null.

My question is, is it possible to have the Vehicle's employee persisted at the same time the Employee itself is being persisted? I realise that the Employee would need to be saved first, then the Vehicle saved afterwards. Can JPA do this automatically for me? Or do I have to do something like the following:

Vehicle vehicle1 = new Vehicle();
Set<Vehicle> vehicles = new HashSet<Vehicle>();
vehicles.add(vehicle1);

Employee newEmployee = new Employee("matt");
newEmployee.setVehicles(vehicles);
Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

vehicle1.setAssociatedEmployee(savedEmployee);
vehicleDao.persistOrMerge(vehicle1);

Thanks!

Edit: As requested, here's my mappings (without all the other methods etc.)

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="employee_id")
    private Long id;

    @OneToMany(mappedBy="associatedEmployee", cascade=CascadeType.ALL)
    private Set<Vehicle> vehicles;

    ...

}

@Entity 
public class Vehicle {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="vehicle_id")
    private Long id;

    @ManyToOne
    @JoinColumn(name="employee_id")
    private Employee associatedEmployee;

    ...
}

I just realised I should have had the following method defined on my Employee class:

public void addVehicle(Vehicle vehicle) {
    vehicle.setAssociatedEmployee(this);
    vehicles.add(vehicle);
}

Now the code above will look like this:

Vehicle vehicle1 = new Vehicle();

Employee newEmployee = new Employee("matt");
newEmployee.addVehicle(vehicle1);
Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

Much simpler and cleaner. Thanks for your help everyone!

Answer 1

Hi.

One way to do that is to set the cascade option on you "One" side of relationship:

class Employee { ...

@OneToMany(cascade = {CascadeType.PERSIST}) private Set vehicles = new HashSet();

... }

by this, when you call

Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

it will save the vehicles too.

BR

Denes

Answer 2

You have to set the associatedEmployee on the Vehicle before persisting the Employee.

Employee newEmployee = new Employee("matt");
vehicle1.setAssociatedEmployee(savedEmployee);
vehicles.add(vehicle1);

newEmployee.setVehicles(vehicles);

Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);