title: How to properly avoid JOINs to count associated elements in Django?
body: <p>In a "many to one" relationship, is it a good Django practice to store the count of related elements on the "one" side, or Django had a better mechanism reducing the overhead ? 
  Say we have two tables : "Posts" and "Likes" and we can have several likes on one post. My DBA instinct is to add a nb_likes field on "Posts" and increment it in the Like(post_id,user_id...) method. The default Django ORM behavior is to make a "JOIN" which seems to be very wasteful.</p>