title: Find all objects with no associated has_many objects
body: <p>In my online store, an order is ready to ship if it in the "authorized" state and doesn't already have any associated shipments. Right now I'm doing this:</p>
  
  <pre><code>class Order &lt; ActiveRecord::Base
      has_many :shipments, :dependent =&gt; :destroy
  
      def self.ready_to_ship
          unshipped_orders = Array.new
          Order.all(:conditions =&gt; 'state = "authorized"', :include =&gt; :shipments).each do |o|
              unshipped_orders &lt;&lt; o if o.shipments.empty?
          end
          unshipped_orders
      end
  end
  </code></pre>
  
  <p>Is there a better way?</p>