JPA many-to-one relation - need to save only Id

Question

I have 2 classes: Driver and Car. Cars table updated in separate process. What I need is to have property in Driver that allows me to read full car description and write only Id pointing to existing Car. Here is example:

@Entity(name = "DRIVER")
public class Driver {
... ID and other properties for Driver goes here .....

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "CAR_ID")
    private Car car;

    @JsonView({Views.Full.class})
    public Car getCar() {
      return car;
    }
    @JsonView({Views.Short.class})
    public long getCarId() {
      return car.getId();
    }
    public void setCarId(long carId) {
      this.car = new Car (carId);
    }

}

Car object is just typical JPA object with no back reference to the Driver.

So what I was trying to achieve by this is:

1. I can read full Car description using detailed JSON View
2. or I can read only Id of the Car in Short JsonView
3. and most important, when creating new Driver I just want to pass in JSON ID of the car. This way I dont need to do unnesessery reads for the Car during persist but just update Id.

Im getting following error:

object references an unsaved transient instance - save the transient instance before flushing : com.Driver.car -> com.Car

I dont want to update instance of the Car in DB but rather just reference to it from Driver. Any idea how to achieve what I want?

Thank you.

UPDATE: Forgot to mention that the ID of the Car that I pass during creation of the Driver is valid Id of the existing Car in DB.

Answer 1

You can do this via getReference call in EntityManager:

EntityManager em = ...;
Car car = em.getReference(Car.class, carId);

Driver driver = ...;
driver.setCar(car);
em.persist(driver);

This will not execute SELECT statement from the database.