[FIXED] Save entities in Spring Data JPA which contains a list of another entity type

Issue

Guys I'm new to Spring so bear with me. I have two entities. User and Genre. User entity contains a list of Genre entities. i.e. "User has genres" ("Genre has users" is not a requirement of me.)

My two entities...

@Entity
@Table(name = "user")
@Getter
@Setter
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class User {
    @Id
    @GeneratedValue
    private int id;
    private String username;
    private String userEmail;
    private String firstName;
    private String lastName;
    private String imgUrl;

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "user_genres", joinColumns = @JoinColumn(name = "User_id"), inverseJoinColumns = @JoinColumn(name = "genres_id"))
    private List<Genre> genres;
  
}

@Data
@Entity
@NoArgsConstructor
@AllArgsConstructor
@Getter
@Setter
public class Genre {
    @Id
    @GeneratedValue
    private int id;
    private String name;


}

and this is the request body I'm sending to save my User record...

{
   "username":"test username",
   "userEmail":"[email protected]",
   "firstName":"test fname",
   "lastName":"test lname",
   "imgUrl":"url",
   "genres":[
      {
         "id":8,
         "name":"Drama"
      }
   ]
}

I maintain a table of genres. When I want to save a user, I pick some genres from the table, save them in a list in User entity and send them to save(). My expectation is to save those relationships in user_genres table with user_id and genre_id.

My problem is, when I try to save a user with a genre object, if there is already a user with a genre object of id as same as in the user I'm trying to save, it gives me an exception saying that id of genre already exists.

i.e. Suppose I already have a user which contains a genre of id 8. Now I'm sending a new user, that means with a new user_id, which also contains a genre with id 8. Then it will give me an exception,

SQLIntegrityConstraintViolationException: Duplicate entry '8' for key 'UK_1ro2jvu91tg2xsrdye9sk9j1q'

This works fine if I use a different genre with a different id. Please tell me a way to obtain the result as I expected. Thanks.

My repository,

public interface UserRepository extends JpaRepository<User, Integer> {}

My service,

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    private UserRepository userRepository;

    @Override
    public ResponseEntity save(User user) {
        if (getUserByUserEmail(user.getUserEmail()) == null)
            return new ResponseEntity(userRepository.save(user), HttpStatus.OK);
        return new ResponseEntity(null, HttpStatus.BAD_REQUEST);
    }

    public User getUserByUserEmail(String userEmail) {
        return userRepository.findByUserEmail(userEmail);
    }
}

My controller,

@Autowired
    UserServiceImpl userService;


    @PostMapping("/users")
    public ResponseEntity save(@RequestBody User user) {

        return userService.save(user);
    }

Update : Cascade type changed to MERGE In User,

 @ManyToMany(fetch = FetchType.LAZY, cascade=CascadeType.MERGE)
 @JoinTable(name = "user_genres", joinColumns = @JoinColumn(name = "User_id"), 
 inverseJoinColumns = @JoinColumn(name = "genres_id"))
    private List<Genre> genres;

Update : Overrided following methods inside User and Genre

In User,

@Override

    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof User)) return false;
        User user = (User) o;
        return Objects.equals(getId(), user.getId());
    }

    @Override
    public int hashCode() {
        return Objects.hash(getId());
    
}

In Genre,

@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Genre)) return false;
        Genre genre = (Genre) o;
        return Objects.equals(getId(), genre.getId());
    }

    @Override
    public int hashCode() {
        return Objects.hash(getId());
    }

Solution

Step 1

Create hashcode() and equals() methods for both User and Genre based on their Id fields.

Step 2

 @ManyToMany(fetch = FetchType.LAZY, cascade=CascadeType.MERGE)
 @JoinTable(name = "user_genres", joinColumns = @JoinColumn(name = "User_id"), 
 inverseJoinColumns = @JoinColumn(name = "genres_id"))
    private List<Genre> genres;

Now you should be ready to go.

Answered By - Panagiotis Bougioukos