Found shared references to a collection many to many relation

Question

I have this method:

@Override 
public Movie createMovie(Movie movie) {

    Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
    JwtUser user = (JwtUser)authentication.getPrincipal();
    User current_user = userRepository.findOne(user.getId());       

    movieRepository.save(movie);

    userRepository.save(new HashSet<User>(){{
        add(new User(current_user, new HashSet<Movie>(){{
            add(movie);
        }}));
    }});

    return movieRepository.save(movie);
}

When I run my application and call that function I get this error:

Found shared references to a collection: com.movieseat.model.security.User.movies

In my User model I have:

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

And in my Movie model I have:

@ManyToMany(mappedBy = "movies", cascade = CascadeType.ALL)
private Set<User> users = new HashSet<>(); 

public Set<User> getUsers() {
    return users;
}

What produces the error?

Answer 1

As I understand your code, you're trying to create a Movie in database and bind it to the current User. Correct me if I'm wrong.

At first, as you may learn from Hibernate User Guide, bidirectional @ManyToMany association should be defined and used differently.

A bidirectional @ManyToMany association has an owning and a mappedBy side. To preserve synchronicity between both sides, it’s good practice to provide helper methods for adding or removing child entities.

Secondly, you should not use CascadeType.ALL on @ManyToMany associations:

For @ManyToMany associations, the REMOVE entity state transition doesn’t make sense to be cascaded because it will propagate beyond the link table. Since the other side might be referenced by other entities on the parent-side, the automatic removal might end up in a ConstraintViolationException.

For example, if @ManyToMany(cascade = CascadeType.ALL) was defined and the first person would be deleted, Hibernate would throw an exception because another person is still associated with the address that’s being deleted.

So, we should move cascade to the owning side, change cascade type, provide helper methods to the User and update only the owning side (User) of the association in our business logic. Let's change the code.

User model:

@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
@JoinTable(name = "user_movie",
    joinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id"),
    inverseJoinColumns = @JoinColumn(name = "movie_id", referencedColumnName = "id")
)
private Set<Movie> movies = new HashSet<>();

public void addMovie(Movie movie) {
    movies.add(movie);
    movie.getUsers().add(this);
}

public void removeMovie(Movie movie) {
    movies.remove(movie);
    movie.getUsers().remove(this);
}

Movie model:

@ManyToMany(mappedBy = "movies")
private Set<User> users = new HashSet<>();

And business logic:

@Override
public Movie createMovie(Movie movie) {

    Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
    JwtUser user = (JwtUser)authentication.getPrincipal();
    User current_user = userRepository.findOne(user.getId());

    current_user.addMovie(movie);
    userRepository.save(current_user);

    return movie;
}