[ACCEPTED]-Can add extra field(s) to @ManyToMany Hibernate extra table?-hibernate

If you add extra fields on a linked table 13 (STUDENT_COURSE), you have to choose an 12 approach according to skaffman's answer 11 or another as shown bellow.

There is an approach 10 where the linked table (STUDENT_COURSE) behaves 9 like a @Embeddable according to:

@Embeddable
public class JoinedStudentCourse {

    // Lets suppose you have added this field
    @Column(updatable=false)
    private Date joinedDate;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="STUDENT_ID", insertable=false, updatable=false)
    private Student student;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="COURSE_ID", insertable=false, updatable=false)
    private Course course;

    // getter's and setter's 

    public boolean equals(Object instance) {
        if(instance == null)
            return false;

        if(!(instance instanceof JoinedStudentCourse))
            return false;

        JoinedStudentCourse other = (JoinedStudentCourse) instance;
        if(!(student.getId().equals(other.getStudent().getId()))
            return false;

        if(!(course.getId().equals(other.getCourse().getId()))
            return false;

        // ATT: use immutable fields like joinedDate in equals() implementation
        if(!(joinedDate.equals(other.getJoinedDate()))
            return false;

        return true;
    }

    public int hashcode() {
        // hashcode implementation
    }

}

So you will 8 have in both Student and Course classes

public class Student {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="STUDENT_COURSE"),
        joinColumns=@JoinColumn(name="STUDENT_ID")
    )
    private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();

}

public class Course {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="STUDENT_COURSE"),
        joinColumns=@JoinColumn(name="COURSE_ID")
    )
    private Set<JoinedStudentCourse> joined = new HashSet<JoinedStudentCourse>();

}

remember: @Embeddable 7 class has its lifecycle bound to the owning 6 entity class (Both Student and Course), so 5 take care of it.

advice: Hibernate team suppports 4 these two approachs (@OneToMany (skaffman's 3 answer) or @CollectionsOfElements) due some 2 limitations in @ManyToMany mapping - cascade 1 operation.

regards,

The student_course table is there purely 9 to record the association between the two 8 entities. It is managed by hibernate, and 7 can contain no other data.

The sort of data 6 you want to record needs to be modelled 5 as another entity. Perhaps you could a one-to-many 4 association between Course and StudentResult 3 (which contains the grade, etc), and then 2 a many-to-one association between StdentResult 1 and Student.

Drop the many-to-many, create a class called 5 StudentCourseRelationship and set up one 4 to manys on Student and Course to the StudentCourseRelationship.

You 3 can put all sorts of things on it, like 2 DateEnrolled, DateKickedOut etc. etc.

IMO 1 the many-to-many mapping is a bit of a con.

The accepted answer unfortunately doesn't 5 work for me, hibernate generates the join 4 table in a weird way (all join columns are 3 duplicated). However the variant with dedicated 2 entity for the join table works fine. Here 1 it is described in great detail: http://www.mkyong.com/hibernate/hibernate-many-to-many-example-join-table-extra-column-annotation/