Fix line item cache migration to work across orders in the staging servers.

db/migrate/20130805050109_update_line_item_caching.rb

@@ -2,11 +2,17 @@ class UpdateLineItemCaching < ActiveRecord::Migration
 
   class SpreeLineItem < ActiveRecord::Base
     belongs_to :shipping_method, class_name: 'Spree::ShippingMethod'
+    belongs_to :variant, :class_name => "Spree::Variant"
 
     def itemwise_shipping_cost
       order = OpenStruct.new :line_items => [self]
       shipping_method.compute_amount(order)
     end
+
+    def amount
+      price * quantity
+    end
+    alias total amount
   end