Java.Hibernate.Middle.How do you map a many-to-many relationship with extra columns?

✅ Short Answer

To map a many-to-many relationship with extra columns in the join table, you can’t use plain @ManyToMany.
Instead, you must model the join table as a separate entity with two @ManyToOne relationships — this is called an association entity (or link entity).

🔎 Detailed Explanation

🔹 Why doesn’t @ManyToMany work?

• JPA’s @ManyToMany creates a join table, but doesn’t let you add extra columns — it only maps the two foreign keys.
• As soon as you need attributes in the join table (e.g., quantity, timestamp), you must model it explicitly.

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🧑‍💻 Example: Students enrolled in Courses

You want to store enrollmentDate in the join table between Student and Course.

@Entity
public class Enrollment {
    @EmbeddedId
    private EnrollmentId id;

    @ManyToOne
    @MapsId("studentId")
    private Student student;

    @ManyToOne
    @MapsId("courseId")
    private Course course;

    private LocalDate enrollmentDate; // extra column!
}

Composite Key for Enrollment

@Embeddable
public class EnrollmentId implements Serializable {
    private Long studentId;
    private Long courseId;

    // equals() and hashCode() required!
}

Entities with mapped enrollments

@Entity
public class Student {
    @Id
    private Long id;

    @OneToMany(mappedBy = "student", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Enrollment> enrollments = new ArrayList<>();
}

@Entity
public class Course {
    @Id
    private Long id;

    @OneToMany(mappedBy = "course", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Enrollment> enrollments = new ArrayList<>();
}

🔹 What does this give you?
✅ A join table represented by Enrollment with:

• student_id (FK)
• course_id (FK)
• enrollment_date (extra column)
  ✅ Ability to attach attributes to the relationship → essential for real-world scenarios.

📊 Benefits of association entity approach

✅ Adds flexibility → store relationship metadata.
✅ Supports cascade operations & orphan removal on relationships.
✅ Allows proper object modeling → no awkward DTO hacks.

📌 Key Takeaways

✅ You can’t use @ManyToMany directly with extra columns → must create an association entity.
✅ Map the association entity with @ManyToOne + @EmbeddedId (or @IdClass) for composite keys.
✅ This pattern gives you full power over join table attributes.