Hibernate Many-To-One Relationship without Foreign Key but with reverse foreign key

Question

I have the following DB:

CREATE TABLE car_owner (
  car_owner_id int(11) NOT NULL,
  car_id_fk int(11) DEFAULT NULL,
  PRIMARY KEY (car_owner_id),
  KEY car_owner_car_fk_idx (car_id_fk),
  CONSTRAINT car_owner_car_fk FOREIGN KEY (car_id_fk) REFERENCES car (car_id) ON DELETE NO ACTION ON UPDATE NO ACTION,
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


CREATE TABLE car (
   car_id int(11) NOT NULL AUTO_INCREMENT,
   car_type varchar(45) DEFAULT NULL,
   car_plates varchar(25) DEFAULT NULL,
  PRIMARY KEY (car_id),
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;

And In the java model:

For CarOwner I have:

@Entity
@Table(name="car_owner")
@NamedQuery(name="CarOwner.findAll", query="SELECT co FROM CarOwner co")
public class CarOwner implements Serializable {

    @Id
    @GeneratedValue
    @Column(name="car_owner_id")
    private Integer carOwnerId;
.....

    //bi-directional many-to-one association to Car
    @OneToMany(fetch = FetchType.EAGER)
    @JoinColumn(name = "car_id_fk", referencedColumnName = "car_id")
    private List<Car> cars;

And for Car:

@Entity
@Table(name="car")
@NamedQuery(name="Car.findAll", query="SELECT c FROM Car c")
public class Car implements Serializable {

    @Id
    @GeneratedValue
    @Column(name="car_id")
    private Integer carId;
......

    //bi-directional many-to-one association to car_owner
    @ManyToOne(fetch=FetchType.EAGER)
    @JoinColumn(name = "car_owner_id")
    private CarOwner carOwner;

The problem here is that Hibernate can't relate the tables and goes creating new car_id and car_owner_id columns in car table automatically.

Can anybody help in finding the right combination in the model to relate the tables appropriately.

Answer 1

@JoinColumn should be in owner of relationship (in a one to many it's the many side that's regarded the owner).

So I will modify this in car

 @ManyToOne(fetch=FetchType.EAGER)
 @JoinColumn(name = "car_owner_id",insertable=false, updatable=false)
 private CarOwner carOwner;

And this in CarOwner

@OneToMany(fetch = FetchType.EAGER,mappedBy = "carOwner")
 private List<Car> cars;

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As a side note I would also not be using EAGER but that has got nothing to do with question.

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Both tables knowing about each other is called Bi-directional relationship.This happens when each table has a key to other table. This is what your java code is expecting.Your tables in database however have a uni-directional relationship. Meaning one table knows about the other but not both. Your car_owner knows about car because of foriegn key CONSTRAINT car_owner_car_fk FOREIGN KEY but your car does not have any idea about car_owner both are perfectly valid.

Now the problem is that in your Java code you are treating it as a bi-directional relationship.

//bi-directional many-to-one association to car_owner
 @ManyToOne(fetch=FetchType.EAGER)
 @JoinColumn(name = "car_owner_id")
 private CarOwner carOwner;

but car does not have car_owner_id why are you treating it a Bi-directional relationship.

Now either update database to make them bi or change java code.