Hibernate Mapping - How to Join Three Tables

Question

I have 3 Entity: PERSON, CAR, and PROFFESION.

    @Entity
    @Table(name = "PERSON")
    public class Person implements Serializable {

        @Id
        private Long id;

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

        @OneToMany(mappedBy = "person", fetch = FetchType.EAGER)
        private List<Profession> professions;
    }

    @Entity
    @Table(name = "PROFESSION")
    public class Profession implements Serializable {

        @Id
        private Long id;

        @ManyToOne
        @JoinColumn(name = "PERSON_ID")
        private Person person;
    }

    @Entity
    @Table(name = "CAR")
    public class Car implements Serializable {

        @Id
        private Long id;

        @ManyToOne
        @JoinColumn(name = "PERSON_ID")
        private Person person;
    }

When i'm trying to retrieve a person with connections to two professions and one car the result is two professions and two duplicated cars.

Or if he is connected to five cars and one profession, the result is five cars and five duplicated professions.

How should I correct the mappings so I receive the right results?

Simplest options. Remove eager fetch or change List<> to Set<>. — Alan Hay, May 25 '16 at 07:27
Thanks, I replaced List with Set and it worked. Yes, i use JPA. — Vako Beridze, May 25 '16 at 08:11
@VakoBeridze It is interesting, that with `Hibernate` (not using `JPA`) you can't get `Person` with `List` at all, cause of `MultipleBagFetchException` :) — v.ladynev, May 25 '16 at 08:15

score 2 · Accepted Answer · answered May 25 '16 at 07:43

2

Use a Set to map oneToMany If you don't want duplicates. Its mapped with the <set> element in mapping table. so first you make changes to these parts:

private Set<Car> car = new HashSet<Car>(0);

@OneToMany(fetch=FetchType.LAZY, mappedBy="persons")
    public Set<Car> getCar() {
        return this.car;
 }
public void setCar(Set<Car> car) {
        this.car = car;
    }

Do the same with profession and another oneToMany you do not want duplicates. you can set fetchType based on loading preferences. Eager load all at once and Lazy load on demand which is usually best.

answered May 25 '16 at 07:43

luffy

70
10

Thanks, I replaced List with Set, and it worked. But I think this is solving the problem on code level. How would I fix it on the database level? – Vako Beridze May 25 '16 at 08:09
2

@VakoBeridze This duplicates because of the `left outer join` in the `SQL`. You can change this behaviour by using subselects (using `JPQL`), but, probably, you lost some performance. – v.ladynev May 25 '16 at 08:20
@v.ladynev ok. thanks a lot. – Vako Beridze May 25 '16 at 08:23
@VakoBeridze You are welcome – v.ladynev May 25 '16 at 08:27

Hibernate Mapping - How to Join Three Tables

1 Answers1