Add a new value on a relation list between users and a table

SOLVED

The situation is that I have two tables, the table User that CloudBoost gives you, and the table Movie that I made up. User contains a list called movies for the relation, being a relation many to many.

What I want to make is to add a new movie to the list of movies that the user already have. That's what I thought it should work:

var user = CB.CloudUser.current;
var movies = user.get("movies");
movies.push(movie);
user.set('movies', movies);
user.save({
  success : function(user){
    //do things
  }, error : function(err){
    //do things
  }
});

However, it cannot make the user.save, it bumps an error.

How could I made this? Thanks.

The solution that I came up with was simply to put the relation list on the Movie table and not on the User one. This implies more queries and more code, but at the end it's what works.