title: rails3 scope for count of children in has_many relationship
body: <p>trying to do a scope in rails3. </p>
  
  <pre><code>:book has_many :chapters 
  </code></pre>
  
  <p>I want scope :long to return books with > 10 chapters.</p>
  
  <p>How best to structure this scope (without use of counter cache) ?</p>
  
  <p>thanks!</p>