title: Update many to many relationship having ids
body: <p>I have 3 model with many to many relationship:</p>
  
  <pre><code>Movie
  Person
  MovieToPerson
  </code></pre>
  
  <p>Say I have a single person and I want to update the list of movies they star in. For movies I have ids. How can I do that?</p>
  
  <pre><code>pers = Person.first
  movie_ids = get_movies_ids
  pers.movies = ....???? 
  </code></pre>
  
  <p>I want to avoid querying the movie objects before updating.</p>