Unidirectional ManyToMany readonly relationship

Question

I have an entity Student and an entity Course. One student can be associated to 0 or more Courses. Viceversa, one Course can have associated 0 or more Students.

Student entity:

@Data
@Entity(name = "student")
public class Student {

    @Id
    private Integer id;

    private String name;

    @ManyToMany(fetch = EAGER)
    @JoinTable(name = "student_course",
            joinColumns = @JoinColumn(
                    name = "studentId",
                    referencedColumnName = "id",
                    insertable = false,
                    updatable = false
            ),
            inverseJoinColumns = @JoinColumn(
                    name = "courseId",
                    referencedColumnName = "id",
                    insertable = false,
                    updatable = false)
    )
    private Collection<Course> courses;
}

Course entity:

@Data
@Entity(name = "course")
public class Course {

    @Id
    private Integer id;

    private String name;

    @ManyToMany(mappedBy = "courses")
    private Collection<Student> students;
}

and the inverse association in the entity Course.

Both of those 2 @ManyToMany associations should be readonly. My problem is that when I try to save a Student Hibernate tries to update also the associated collection.

Here's what hibernate logs on a student update:

Hibernate: 
/* delete collection model.student.courses */ 
delete from `student_course` 
where `studentId`=?

Hibernate: 
/* insert collection row  */
insert into `student_course` (`studentId`, `courseId`) 
values (?, ?)

As you can see hibernate is trying to update also the table that stores the associations between the two entities. Those are the queries that I want to avoid.

Answer 1

Student class:

@ManyToMany(fetch = LAZY)
@JoinTable(
    name="STUDENT_COURSE"
    , joinColumns={
        @JoinColumn(name="STUDENT_ID")
        }
    , inverseJoinColumns={
        @JoinColumn(name="COURSE_ID")
        }
    )
private List<Course> courses;

Course class :

@ManyToMany(mappedBy="courses")
private List<Student> students;

As you khnow, from the JPA 2.0 spec, the defaults fetch are:

OneToMany: LAZY
ManyToOne: EAGER
ManyToMany: LAZY
OneToOne: EAGER

And in the latest version of hibernate fetch type is eager by deafult for all the mappings but if we are using JPA annotations then it aligns with JPA defaults.

Answer 2

The issue is that a list is ordered whereas a set is not. The generalized code for updating a list seems to assume that you wish to preserve that order and so it must delete the entire List of relations and reinsert it fresh to preserve the order through the relation PK ids. This would be explicitly true if you added an @OrderBy annotation on the join property. In a set this is not necessary so it can just do an insert.

@Entity
@Data
@EqualsAndHashCode(of="id")
@NoArgsConstructor
public class E1 {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @ManyToMany
    Set<E2> e2s;

@Entity
@Data
@EqualsAndHashCode(of="id")
@NoArgsConstructor
public class E2 {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @ManyToMany(mappedBy="e2s")
    Set<E1> e1s;

And to use them:

E1 e1 = new E1();
E2 e2 = new E2();
Set<E2> es2 = new HashSet<>();
es2.add(e2);
e1.setE2s(es2);

em.persist(e1);
em.persist(e2);
tx.commit();

// now modify the list
em.clear();

// fetch relations to avoid lazy fetch exception
e1 = em.createQuery("select e from E1 e left outer join fetch e.e2s where e.id = :id", E1.class)
        .setParameter("id", 1L)
        .getSingleResult();

tx.begin();
e2 = new E2();
e1.getE2s().add(e2);
em.persist(e2);
em.merge(e1);
tx.commit();

This gives me the following logs

create table E1_E2 (e1s_id bigint not null, e2s_id bigint not null, primary key (e1s_id, e2s_id))
create table E1 (id bigint generated by default as identity (start with 1), primary key (id))
create table E2 (id bigint generated by default as identity (start with 1), primary key (id))
alter table E1_E2 add constraint FKky9vffxlkk0u9t0ynqfsvanrt foreign key (e2s_id) references E2
alter table E1_E2 add constraint FKgnbwe4qtab0mt1caqxrrp8gqd foreign key (e1s_id) references E1
insert into E1 (id) values (default)
insert into E2 (id) values (default)
insert into E1_E2 (e1s_id, e2s_id) values (?, ?)
select e1x0_.id as id1_2_0_, e2x2_.id as id1_4_1_, e2s1_.e1s_id as e1_3_0__, e2s1_.e2s_id as e2_3_0__ from E1 e1x0_ left outer join E1_E2 e2s1_ on e1x0_.id=e2s1_.e1s_id left outer join E2 e2x2_ on e2s1_.e2s_id=e2x2_.id where e1x0_.id=?
insert into E2 (id) values (default)
insert into E1_E2 (e1s_id, e2s_id) values (?, ?)