title: cross model scoping - rails 3
body: <p>I want to create a scope for all Posts without comments... I do not understand how, in the model (by creating a scope), I can check if my Post has any Comments attached to it as only the Comments seem to know what Post they belong to, as opposed to the Post knowing what Comments belong to it.</p>
  
  <pre><code>Post
  has_many :comments
  
  Comments
  belong_to :post
  </code></pre>
  
  <p>(Please stop me if I'm wrong.)</p>