Java.Hibernate.Beginner.What is many-to-many mapping in Hibernate?

A many-to-many mapping models a relationship where multiple instances of one entity can relate to multiple instances of another entity, and vice versa — like a student can enroll in many courses, and a course can have many students.

In relational databases, many-to-many relationships always require a join table, since neither side can hold a simple foreign key to represent multiple references.

---

🔹 Real-world examples:

• Students ↔ Courses
• Users ↔ Roles
• Authors ↔ Books

---

🔹 How to map many-to-many in Hibernate (JPA annotations):
You use @ManyToMany on both entities, optionally with @JoinTable to customize the join table details.

---

✅ Example: Student ↔ Course

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToMany
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private List<Course> courses = new ArrayList<>();

    // getters and setters
}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;

    @ManyToMany(mappedBy = "courses")
    private List<Student> students = new ArrayList<>();

    // getters and setters
}

🔹 What happens in the database:
Hibernate automatically creates a join table like:

student_course
------------------------
student_id | course_id

Each row in this join table links one student to one course, enabling the many-to-many association.

🔹 Bidirectional mapping:

• The mappedBy attribute in Course.students points to the courses field in Student.
• This tells Hibernate which side owns the relationship and avoids creating two join tables.

🔹 How to use it in code:

Student student = new Student();
student.setName("Alice");

Course course1 = new Course();
course1.setTitle("Math");

Course course2 = new Course();
course2.setTitle("Physics");

// Link both sides of the relationship
student.getCourses().add(course1);
student.getCourses().add(course2);
course1.getStudents().add(student);
course2.getStudents().add(student);

session.persist(student); // cascades can optionally persist courses

🔹 Key points:
✅ Join table is required for all many-to-many mappings → no direct foreign key can represent it.
✅ Supports both unidirectional (one side knows about the other) and bidirectional (both sides know about each other) mappings.
✅ Can customize the join table name and columns using @JoinTable.

✅ Key takeaway:
Many-to-many mapping in Hibernate models associations where multiple entities on both sides can relate to each other, using a join table mapped with @ManyToMany.