Spring Boot: JPA Many-to-Many Mapping

Introduction

When designing relational databases in Java, you’ll often encounter situations where multiple entities are related to multiple other entities. This is where Many-to-Many mapping comes into play!

By the end of this article, you’ll learn:

• What Many-to-Many mapping is.
• How to implement it using Spring Boot, JPA, and Hibernate.
• Different ways to set up Many-to-Many relationships.
• Real-world examples to help you apply it in your projects.

Let’s dive in! 🚀

What is Many-to-Many Mapping?

A Many-to-Many relationship means that one entity is related to multiple entities, and vice versa.

Real-World Examples:

• A student can enroll in multiple courses, and a course can have multiple students.
• An author can write multiple books, and a book can have multiple authors.
• A product can belong to multiple categories, and a category can have multiple products.

To achieve this, we need a join table that links the two entities together.

Let’s implement this using JPA and Spring Boot! 🔥

Implementing Many-to-Many Mapping in Spring Boot

1️⃣ Many-to-Many Using a Join Table (Recommended ✅)

📌 Step 1: Create the Student Entity

import jakarta.persistence.*;
import java.util.List;

@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;

    // Getters and Setters
}

📌 Step 2: Create the Course Entity

import jakarta.persistence.*;
import java.util.List;

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

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

    // Getters and Setters
}

🔹 Explanation:

• The @ManyToMany annotation defines a many-to-many relationship.
• The @JoinTable in Student specifies the join table (student_course).
• The joinColumns and inverseJoinColumns define the foreign keys for both entities.
• In Course, we use mappedBy = "courses" to indicate that the relationship is managed by Student.

💡 Tip: JPA will automatically create the student_course table with student_id and course_id as foreign keys.

2️⃣ Many-to-Many with an Extra Column in Join Table 📊

If you need to store additional data in the join table (e.g., grade for a student in a course), you should create a separate mapping entity.

📌 Step 1: Create the StudentCourse Entity (Join Table)

import jakarta.persistence.*;

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

    @ManyToOne
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne
    @JoinColumn(name = "course_id")
    private Course course;

    private String grade;

    // Getters and Setters
}

📌 Modify Student and Course

@OneToMany(mappedBy = "student")
private List<StudentCourse> studentCourses;

@OneToMany(mappedBy = "course")
private List<StudentCourse> studentCourses;

🔹 Why use this approach?

• You can store extra details like grades, enrollment date, etc..
• The StudentCourse entity acts as a bridge between Student and Course.

When to Use Many-to-Many Mapping?

Many-to-Many relationships are useful when:
✅ Entities have multiple associations with each other (e.g., students & courses).
✅ You want to avoid duplicate data while maintaining flexibility.
✅ You need to model a complex relationship with extra attributes (e.g., grades in courses).

Conclusion 🎯

In this article, we explored:

• What Many-to-Many mapping is.
• How to implement it in Spring Boot using JPA.
• Different approaches (basic join table vs. additional attributes).
• When to use it in real-world applications.

Many-to-Many relationships help model complex data structures efficiently. Mastering them will take your backend skills to the next level! 🚀

💬 Got questions? Let me know in the comments! Happy coding! 😃