title: Rails find where HABTM association is nil
body: <p>I have <code>Popup</code> and <code>User</code> that have a HABTM association. </p>
  
  <p>I am trying to write a scope for <code>Popup</code> that would look like: <code>Popup.not_seen_by(current_user).first</code></p>
  
  <p>Unfortunately, my implementation below doesn't work.  I feel like I'm on the right track, would anyone be willing to help me get over this last hurdle?</p>
  
  <pre><code>class Popup &lt; ActiveRecord::Base
    has_and_belongs_to_many :users, :uniq =&gt; true
  
    scope :not_seen_by, lambda { |user| 
      # This does NOT work
      joins("LEFT JOIN popups_users ON popups.id = popups_users.popup_id").
      where("popups_users.user_id != ?", user.id)
    }
  end
  </code></pre>
  
  <p>Thank you!</p>