title: ActiveRecord: Find without associations
body: <p>I have a <code>many_to_many</code> relationship between ImageShells and Users.  I need to find all ImageShells that have no users.   </p>
  
  <p>I have a query that finds them but how do I put this into a <code>named_scope</code>?</p>
  
  <pre><code>SELECT * FROM image_shells INNER JOIN image_shells_users ON (image_shells_users.image_shell_id!=image_shells.id)
  
  
  class ImageShell &lt; ActiveRecord::Base
     has_and_belongs_to_many :riders, :class_name =&gt; "User"
  end
  
  class User &lt; ActiveRecord::Base  
    has_and_belongs_to_many :image_shells
  end
  </code></pre>
  
  <p>I'm able to use find by sql but this is messy.</p>
  
  <pre><code>img_shells_with_out_users = ImageShell.find_by_sql 'SELECT * FROM image_shells INNER JOIN image_shells_users ON (image_shells_users.image_shell_id!=image_shells.id)'
  </code></pre>