title: elixir delete many to many association
body: <p>I have two models: User and Group. They are many to many associated through a join table. 
  When I try to delete a user ( or a group), it raises this error: </p>
  
  <p>** (Ecto.ConstraintError) constraint error when attempting to delete model:</p>
  
  <pre><code>* foreign_key: groups_user_id_key
  </code></pre>
  
  <p>What should I do to delete any of the parent model? </p>