How to annotate Map with JPA?

Question

The app's stack: Spring MVC, Spring DataJPA, Hibernate. There are three entities: student, tutor, theme.

Theme:

@Entity
@Table(name = "themes")
public class Theme {
    // fields omitted
}

Student:

@Entity
@Table(name = "students")
public class Student {

    private Map<Theme, Tutor> tutors;
    // other fields omitted
}

Tutor:

@Entity
@Table(name = "tutors")
public class Tutor {

    private Map<Theme, Student> students;
    // other fields omitted
}

For save student-tutor-theme relationships i want use this table (PostgreSQL):

CREATE TABLE themes_students_tutors
(
    theme_id   INTEGER NOT NULL,
    student_id INTEGER NOT NULL,
    tutor_id   INTEGER NOT NULL,
    FOREIGN KEY (theme_id) REFERENCES themes (id) ON DELETE CASCADE,
    FOREIGN KEY (student_id) REFERENCES students (id) ON DELETE CASCADE,
    FOREIGN KEY (tutor_id) REFERENCES tutors (id) ON DELETE CASCADE
)

How i can to annotate tutors and students fields in entities, for their content correct persists in this table?

Try the [`@MapKeyJoinColumn`](https://stackoverflow.com/a/25447741/1530938). — kolossus, Feb 14 '21 at 17:03
@dan1st oh, maybe. but i not know, how i can use it. I understand the general principle, but I am interested in the specific application of these annotations in my case — Zhenyria, Feb 14 '21 at 17:06

hjoeren · Accepted Answer · 2021-02-14T18:14:52.967

Like @kolossus mentioned: Use the @MapKeyJoinColumn¹ annotation, so that the classes (or the map fields) look like this (you can ignore the extention of AbstractPersistable):

Student:

public class Student extends AbstractPersistable<Long> {

    @ManyToMany
    @JoinTable(name = "themes_students_tutors", joinColumns = {
            @JoinColumn(name = "student_id", referencedColumnName = "id") }, inverseJoinColumns = {
                    @JoinColumn(name = "tutor_id", referencedColumnName = "id") })
    @MapKeyJoinColumn(name = "theme_id")
    private Map<Theme, Tutor> tutors;

}

Tutor:

public class Tutor extends AbstractPersistable<Long> {

    @ManyToMany
    @JoinTable(name = "themes_students_tutors", joinColumns = {
            @JoinColumn(name = "tutor_id", referencedColumnName = "id") }, inverseJoinColumns = {
                    @JoinColumn(name = "student_id", referencedColumnName = "id") })
    @MapKeyJoinColumn(name = "theme_id")
    private Map<Theme, Student> students;

}

Given that, something like this would be created:

Hibernate: create table students (id bigint not null, primary key (id))
Hibernate: create table themes (id bigint not null, primary key (id))
Hibernate: create table themes_students_tutors (tutor_id bigint not null, student_id bigint not null, theme_id bigint not null, primary key (student_id, theme_id))
Hibernate: create table tutors (id bigint not null, primary key (id))
Hibernate: alter table themes_students_tutors add constraint FKm5l4is34t5gs14p4skkv3aup7 foreign key (student_id) references students
Hibernate: alter table themes_students_tutors add constraint FK8o0mm5ywi0l4hdxi4lgw4dbnu foreign key (theme_id) references themes
Hibernate: alter table themes_students_tutors add constraint FKa0n6jvie0kmk0pmikcuvtepxh foreign key (tutor_id) references tutors

¹: See the Javadoc documentation of @MapKeyJoinColumn for some other samples

Didn't know this one, good to know. But anyway, I would map the join table as Entity as I find it easier later on when you have to code some queries for reports. — Delfino Gomes, Feb 14 '21 at 18:19
@DelfinoGomes If it stays a simple join table without additional information, I think it's just fine, not to map it to an entity. But if some information come into play (like additional columns such as "start/end date"), I also think it would be better (or the only possibility) to map the table to an entity. (Good point about queries: I don't know if/how it is possible to use Spring Data features such as query methods like `findByXY`, but I think this would be a separate question) — hjoeren, Feb 14 '21 at 19:23

How to annotate Map with JPA?

1 Answers1