@ManyToMany relationship in Spring Boot - Mini Project: Build & Apply

Create a @ManyToMany Relationship in Spring Boot

📖 Scenario: You are building a simple library system where Books can have multiple Authors, and each Author can write multiple Books.This is a real-world example of a many-to-many relationship.

🎯 Goal: Build two Spring Boot entity classes Book and Author with a proper @ManyToMany relationship between them.

📋 What You'll Learn

Create a Book entity with an id and title field

Create an Author entity with an id and name field

Add a @ManyToMany relationship between Book and Author

Use a join table named book_author with join columns book_id and author_id

💡 Why This Matters

🌍 Real World

Many real-world applications like library systems, social networks, and e-commerce platforms use many-to-many relationships to connect entities.

💼 Career

Understanding @ManyToMany relationships is essential for backend developers working with Spring Boot and relational databases.

1

Create the Book entity

Create a Spring Boot entity class called Book with fields: Long id annotated with @Id and @GeneratedValue, and String title. Annotate the class with @Entity.

Spring Boot

@Entity
public class Book {
    // Your code here
}

Need a hint?

Use @Entity on the class, and add @Id and @GeneratedValue on the id field.

2

Create the Author entity

Create a Spring Boot entity class called Author with fields: Long id annotated with @Id and @GeneratedValue, and String name. Annotate the class with @Entity.

Spring Boot

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

@Entity
public class Author {
    // Your code here
}

Need a hint?

Similar to Book, use @Entity and add @Id and @GeneratedValue on the id field.

3

Add @ManyToMany relationship in Book

In the Book class, add a field Set<Author> authors annotated with @ManyToMany and @JoinTable. Use name = "book_author" for the join table, joinColumns = @JoinColumn(name = "book_id"), and inverseJoinColumns = @JoinColumn(name = "author_id"). Initialize authors as a new HashSet<>().

Spring Boot

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    // Add @ManyToMany authors field here
    // Your code here

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

@Entity
public class Author {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Need a hint?

Use @ManyToMany and @JoinTable with the specified join table and columns. Initialize the authors set.

4

Add @ManyToMany relationship in Author

In the Author class, add a field Set<Book> books annotated with @ManyToMany(mappedBy = "authors"). Initialize books as a new HashSet<>().

Spring Boot

import java.util.HashSet;
import java.util.Set;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.JoinTable;
import jakarta.persistence.ManyToMany;

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    @ManyToMany
    @JoinTable(
        name = "book_author",
        joinColumns = @JoinColumn(name = "book_id"),
        inverseJoinColumns = @JoinColumn(name = "author_id")
    )
    private Set authors = new HashSet();

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public Set getAuthors() {
        return authors;
    }

    public void setAuthors(Set authors) {
        this.authors = authors;
    }
}

@Entity
public class Author {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    // Add @ManyToMany books field here
    // Your code here

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Need a hint?

Use @ManyToMany(mappedBy = "authors") on the books field and initialize it as a new HashSet<>().