Understanding bidirectional relationship from Hibernate documentation

Question

I am going through the Hibernate documentation for bidirectional relationship, in the doc it says that:

Example 7.21. Bidirectional one to many with many to one side as association owner

@Entity
public class Troop {
    @OneToMany(mappedBy="troop")
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk")
    public Troop getTroop() {
    ...
}

Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to (must not) define any physical mapping in the mappedBy side.

To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the mappedBy element and set the many to one @JoinColumn as insertable and updatable to false. This solution is not optimized and will produce additional UPDATE statements.

Example 7.22. Bidirectional association with one to many side as owner

@Entity
public class Troop {
    @OneToMany
    @JoinColumn(name="troop_fk") //we need to duplicate the physical information
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk", insertable=false, updatable=false)
    public Troop getTroop() {
    ...
}

I am finding difficulty in understanding this as I am new to Hibernate.

1) What it means when the doc says: You don't have to (must not) define any physical mapping in the mappedBy side.

2) @JoinColumn in 7.22 has same value (troop_fk) for name attribute. Can we specify different values? What is the advantage & disadvantages of setting insertable=false, updatable=false here?

Can someone please explain?

JB Nizet JB Nizet · Accepted Answer · 2014-08-24T10:08:38

It's a bidirectional association. So, if a soldier balongs to a troop, the troop contains the soldier. These are just the two ways of saying the same thing.

In Soldier, you tell how the association is represented in the database: using a join column named troop_fk:

@JoinColumn(name="troop_fk")
public Troop getTroop() {

So, repeating that same information on the other side of this bidirectional association is redundant. You must not do it. By saying

@OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {

you're telling Hibernate that getSoldiers() is the inverse side of a bidirectional association, and that the way this association is mapped can be found on the Soldier.troop property.

Regarding your second question. Once again, the goal is to define a single, but bidirectional association. You don't need two different foreign keys to map a single association. So specifying a different name for the join column would make no sense: it would create a different, unidirectional association.

This way of doing is an ugly hack that, AFAIK, is not supported by the JPA spec. The JPA spec mandates that the owner side of a bidirectional OneToMany association is the many side. In fact it creates two unidirectional associations mapped the same way, and tells Hibernate (using insertable = false and updatable = false) to ignore one of them when saving the entity. It will populate soldier.troop when reading a soldier from the database, but whatever you put into soldier.troop will be ignored when saving the soldier. You should avoid this way of doing, IMHO.

Understanding bidirectional relationship from Hibernate documentation

1 Answers