title: callback on associated has_many not working
body: <p>Seems like i’ve gone back to basics and am missing something… ..
  I have two models User and Lists. User has many lists and lists belongs to User… When we remove the user from the list, i.e. the lists user_id == nil, we can’t seem to catch the update through a callback like after_update or after_commit.</p>
  
  <pre class="lang-rb prettyprint-override"><code>class List &lt; ApplicationRecord
    after_update :check_user
    belongs_to :user
    private
    def check_user
      binding.pry
      if user_id.blank?
      end
    end
  end
  </code></pre>
  
  <p>we even tried before_save but it doesn’t seem like the model sees the change. Are we missing something obvious??</p>