Many to Many hibernate inverse side ignored

Question

Hi am reading the hibernate documentation.

http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html/entity.html

A many-to-many associ

Answer 1

To make it work both ways you need to have two separate relationships between your entities. This can be represented by one join table in database but by default it will be represented by two so you have to explicitly say you want one join table.

I will demonstrate it using previously mentioned model of Student and Course.

@Entity
public class Student {
    @ManyToMany
    @JoinTable(name = "student_course",
               joinColumns = {@JoinColumn(name = "courses_id")},
               inverseJoinColumns = {@JoinColumn(name = "students_id")})
    private Set courses;
    ...
}

@Entity
public class Course {
    @ManyToMany
    @JoinTable(name = "student_course",
               joinColumns = {@JoinColumn(name = "students_id")},
               inverseJoinColumns = {@JoinColumn(name = "courses_id")})
    private Set students;
    ...
}

In the above example we have 2 relationships with each side of Student<->Course relationship being owner of one relation. So this solves the problem of saving changes to database only on the owner side since each side is owner of one relation.

But we have to keep in mind one fact that after saving data, relationship collections will NOT be reloaded from database so programmer need to handle relationship collections by himself. By saying this I want to say that the easiest way is to modify setters for relationship collections to rebuild cycle between entities like this:

public void setCourses(Set courses) {
    for(Course c : courses) {
        if (!c.getStudents().contains(this)) {
            c.getStudents().add(this);
        }
    }
    this.courses = courses;
}

public void setStudents(Set students) {
    for(Student s : students) {
        if (!s.getCourses().contains(this)){
            s.getCourses().add(this);
        }
    }
    this.students = students;
}