Photo by Everton Vila / Unsplash

In the first part, we saw how to handle a Many-to-Many relationship between two JPA entities without additional. Still, sometimes you might want to store additional information in the association table.

Handle Many-to-Many relationship with JPA in a Spring Boot application - part 1
In this post, we will see how to create many-to-many relationships between entities using JPA and Hibernate ins a Spring Boot project.

In this part, we will see how to handle a Many-to-Many relationship that has additional information. We will continue to use our movie web application and improve the requirement.

The use case

We allowed a user to add movies he likes, and now we want to make it possible to rate the movie, add a review, and finally, the date when the movie has been added. The entity-relation diagram now looks like this:

The updated database schema.
The updated database schema.

Prerequisites

As in the previous part, you will need a MySQL instance installed on your local computer, or Docker must be installed to create a container from the MySQL Docker image. Here is the command to start a container:

docker run -it --rm -e MYSQL_ROOT_PASSWORD=secret -e MYSQL_DATABASE=movied --name mysqldb -p 3307:3306 mysql:8.0

You will also need the tools required to code in Java:

Prepare the Spring Boot project

We will clone the project from part one and then continue to work with:

git clone https://github.com/tericcabrel/blog-tutorials.git

cd blog-tutorials/springboot-many-to-many-1

docker run -it --rm -e MYSQL_ROOT_PASSWORD=secret -e MYSQL_DATABASE=movied --name mysqldb -p 3307:3306 mysql:8.0

mvn install

mvn spring-boot:run

You will get the output below:

Run the project locally

Delete the simple Many-to-Many relationship

With JPA, handling Many-to-Many relationship data that hold additional information is different from the one that doesn't have; before we see how to do it, let's delete the unnecessary code.

In the file User.java of the package entities, remove the code below:

@ManyToMany()
@JoinTable(
    name = "users_movies",
    joinColumns = @JoinColumn(name = "user_id"),
    inverseJoinColumns = @JoinColumn(name = "movie_id")
)
private Set<Movie> movies;

public Set<Movie> getMovies() {
    return movies;
}

public void setMovies(Set<Movie> movies) {
    this.movies = movies;
}

In the file Movie.java of the package entities, remove the code below:

@ManyToMany(mappedBy = "movies")
private Set<User> users;
  
public Set<User> getUsers() {
    return users;
}

Delete all the tables in your database to start with a consistent state.

Create the Many-to-Many relationship

With JPA, when your Many-to-Many relationship between two entities with additional information, we need to create a new Java class, let's say UserMovie.java.

But we have a problem: the primary key is the combination of user_id and movie_id to represent it, we create a Subclass called UserMovieId.java that has the properties userId and movieId. This class will have the annotation @Embeddable

Here is the content of the file UserMovie.java:

package com.tericcabrel.movie.entities;

import java.io.Serializable;
import java.util.Date;
import javax.persistence.Column;
import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Lob;
import javax.persistence.ManyToOne;
import javax.persistence.MapsId;
import javax.persistence.Table;
import org.hibernate.annotations.CreationTimestamp;

@Entity
@Table(name = "users_movies")
public class UserMovie {
  @EmbeddedId
  private UserMovieId id = new UserMovieId();

  @ManyToOne
  @MapsId("userId")
  private User user;

  @ManyToOne
  @MapsId("movieId")
  private Movie movie;

  @Column(nullable = false)
  private int rate;

  @Lob
  private String review;

  @CreationTimestamp
  @Column(name = "added_at", nullable = false)
  private Date addedAt;

  public UserMovie() {}

  public UserMovie(UserMovieId id, int rate, String review) {
    this.id = id;
    this.rate = rate;
    this.review = review;
  }

  public UserMovieId getId() {
    return id;
  }

  public User getUser() {
    return user;
  }

  public Movie getMovie() {
    return movie;
  }

  public int getRate() {
    return rate;
  }

  public String getReview() {
    return review;
  }

  public Date getAddedAt() {
    return addedAt;
  }

  @Embeddable
  public static class UserMovieId implements Serializable {

    private static final long serialVersionUID = 1L;

    private Integer userId;
    private Integer movieId;

    public UserMovieId() {}

    public UserMovieId(Integer userId, Integer movieId) {
      super();
      this.userId = userId;
      this.movieId = movieId;
    }

    public Integer getUserId() {
      return userId;
    }

    public Integer getMovieId() {
      return movieId;
    }

    public void setUserId(Integer userId) {
      this.userId = userId;
    }

    public void setMovieId(Integer movieId) {
      this.movieId = movieId;
    }
  }
}

Run the application with mvn spring-boot:run so Hibernate can create the database tables, and let's verify the result:

View the database schema from a GUI tool.
View the database schema from a GUI tool.

Create the UserMovie repository

Since the table users_movies is now represented by a JPA entity; we need to create his own repository interface. Inside the package repositories, create a file UserMovieRepository.java and add the code below:

package com.tericcabrel.movie.repositories;

import com.tericcabrel.movie.entities.UserMovie;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface UserMovieRepository extends CrudRepository<UserMovie, UserMovie.UserMovieId> {

}

Insert data inside the users_movies table

Let's update the content of the file DataSeeder.java with this one:

package com.tericcabrel.movie;

import com.tericcabrel.movie.entities.Movie;
import com.tericcabrel.movie.entities.User;
import com.tericcabrel.movie.entities.UserMovie;
import com.tericcabrel.movie.entities.UserMovie.UserMovieId;
import com.tericcabrel.movie.repositories.MovieRepository;
import com.tericcabrel.movie.repositories.UserMovieRepository;
import com.tericcabrel.movie.repositories.UserRepository;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.stereotype.Component;

@Component
public class DataSeeder implements ApplicationListener<ContextRefreshedEvent> {
  private final UserRepository userRepository;

  private final MovieRepository movieRepository;

  private final UserMovieRepository userMovieRepository;

  public DataSeeder(UserRepository userRepository, MovieRepository movieRepository, UserMovieRepository userMovieRepository) {
    this.userRepository = userRepository;
    this.movieRepository = movieRepository;
    this.userMovieRepository = userMovieRepository;
  }

  @Override
  public void onApplicationEvent(ContextRefreshedEvent event) {
    Movie movie1 = new Movie("Movie 1", "Movie 1 description", 2020);
    Movie movie2 = new Movie("Movie 2", "Movie 2 description", 2021);

    Movie createdMovie1 = movieRepository.save(movie1);
    Movie createdMovie2 = movieRepository.save(movie2);

    User user = new User("user@email.com", "John Doe");

    User createdUser = userRepository.save(user);

    UserMovie.UserMovieId userMovieId1 = new UserMovieId(createdUser.getId(), createdMovie1.getId());
    UserMovie userMovie1 = new UserMovie(userMovieId1, 4, "This is a good movie");
    userMovie1.setUser(createdUser);
    userMovie1.setMovie(createdMovie1);

    UserMovie.UserMovieId userMovieId2 = new UserMovieId(createdUser.getId(), createdMovie2.getId());
    UserMovie userMovie2 = new UserMovie(userMovieId2, 5, "This is an awesome movie!");
    userMovie2.setUser(createdUser);
    userMovie2.setMovie(createdMovie2);

    userMovieRepository.save(userMovie1);
    userMovieRepository.save(userMovie2);

    Iterable<UserMovie> userMovieList = userMovieRepository.findAll();

    userMovieList.forEach(um -> {
      System.out.println("The user " + um.getUser().getName() + " gave a rate of " + um.getRate() + " to the movie " + um.getMovie().getName());
    });
  }
}

Rerun the application, and you will get the output below:

The data have been inserted successfully
The data have been inserted successfully 🎉

That's it. You can now use Many-to-Many relationships in your Spring Boot project.

You can find the code source on the GitHub repository.

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