How Do I Create Many to Many Hibernate Mapping for Additional Property from the Join Table?

Question

I need a many to many hibernate mapping needed 3 joins. I've tried to find out a solution without intermediate entity like LecturerCourse.

I have a many to many relation in my database between my lecturer and course tables. A course can be given by several lecturer while a lecturer can give several courses.

I have courses stored before hand. However, I need to assign courses to lecturer. When I assign courses I also store the capacity of that course.

My database diagram:

I use hibernate and spring. I need a hibernate mapping when a course is assign any lecturer. I need to add values to capacity field.

My lecturer mapping :

@Entity
@Table(name="LECTURER")
public class Lecturer {

    @Id 
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="LECTURER_ID_SEQ")
    @SequenceGenerator(name="LECTURER_ID_SEQ", sequenceName="LECTURER_ID_SEQ")
    private Long Id;

    @Column(name="NAME")
    private String name;

    @Column(name="SURNAME")
    private String surname;


    @Column(name="EMAIL")
    private String email;

    @Column(name="USERNAME")
    private String username;

    @Column(name="PASSWORD")
    private String Password;

    @ManyToMany
    @JoinTable(
          name="LECTURER_COURSE",
          joinColumns=@JoinColumn(name="LECTURER_ID"),
          inverseJoinColumns=@JoinColumn(name="COURSE_ID")
      )
    private List<Course> courses;

    //getters - setters
}

My course mapping :

@Entity
@Table(name="COURSE")
public class Course {

    @Id 
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="COURSE_ID_SEQ")
    @SequenceGenerator(name="COURSE_ID_SEQ", sequenceName="COURSE_ID_SEQ")
    private Long id;

    @Column(name="NAME")
    private String name;

    @Column(name="CODE")
    private String code;
}

Any idea how to solve my problem ?

Answer 1

You need to use @EmbeddedId and @Embeddable annotations to solve this issue:

Lecturer Class:

@Entity
@Table(name="LECTURER")
public class Lecturer {

@OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.lecturer", cascade=CascadeType.ALL)
Set<LecturerCourse> lecturerCourses == new HashSet<LecturerCourse>();

//all others properties Setters and getters are less relevant.

}

Course class:

@Entity
@Table(name="COURSE")
public class Course {

@OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.course", cascade=CascadeType.ALL)
Set<LecturerCourse> lecturerCourses == new HashSet<LecturerCourse>();

//all others properties Setters and getters are less relevant.

}

LecturerCourse Class:

@Entity
@Table(name = "lecturer_course")
@AssociationOverrides({
        @AssociationOverride(name = "pk.lecturer", 
            joinColumns = @JoinColumn(name = "LECTURER_ID")),
        @AssociationOverride(name = "pk.course", 
            joinColumns = @JoinColumn(name = "COURSE_ID")) })
public class LecturerCourse {

    private LecturerCourseID pk = new LecturerCourseID();

    @Column(name = "CAPACITY", nullable = false, length = 10)
    private String capacity;

    @EmbeddedId
    public LecturerCourseID getPk() {
        return pk;
    }

}

Now the Primary Key:

@Embeddable
public class LecturerCourseID implements java.io.Serializable {

    private Lecturer lecturer;
    private Course course;

    @ManyToOne
    public Stock getLecturer() {
        return lecturer;
    }

    public void setLecturer(Lecturer lecturer) {
        this.lecturer= lecturer;
    }

    @ManyToOne
    public Course getCourse() {
        return course;
    }

    public void setCourse(Course course) {
        this.course= course;
    }

}

now Your Main should be something like this:

Lecturer lecturer1 = new Lecturer();
Course math = new Course();
LecturerCourse lecturer1math  = new LecturerCourse();
lecturer1math.setCapacity("capacity");
lecturer1math.setLecturer(lecturer1);
lecturer1math.setCourse(math);
lecturer1.getLecturerCourses().add(lecturer1math);

//saving object
session.save(lecturer1);

You need to be sure that class marked as @Embeddable should implement Serializable marker interface.

Hope it helps.