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

Suppose you have the following entities:

@Entity
public class Student {
    @ManyToMany
    private Set courses;
    ...
}

@Entity
public class Course {
    @ManyToMany(mappedBy = "courses")
    private Set students;
    ...
}

The owner side is Student (because it doesn't have the mappedBy attribute). The inverse side is Course ((because it has the mappedBy attribute).

If you do the following:

Course course = session.get(Course.class, 3L);
Student student = session.get(Student.class, 4L);
student.getCourses().add(course);

Hibernate will add an entry for student 4 and course 3 in the join table because you updated the owner side of the association (student.courses).

Whereas if you do the following:

Course course = session.get(Course.class, 3L);
Student student = session.get(Student.class, 4L);
course.getStudents().add(student);

nothing will happen, because uou updated the inverse side of the association (course.students), but neglected to updated the owner side. Hibernate only considers the owner side.