Define class used in migration.

db/migrate/20130809045637_create_enterprise_fee_records_for_product_distributions.rb

@@ -1,4 +1,11 @@
 class CreateEnterpriseFeeRecordsForProductDistributions < ActiveRecord::Migration
+
+  class ProductDistribution < ActiveRecord::Base
+    belongs_to :shipping_method, :class_name => 'Spree::ShippingMethod'
+    belongs_to :distributor, :class_name => 'Enterprise'
+    belongs_to :enterprise_fee
+  end
+
   def up
     ProductDistribution.all.each do |pd|
       calculator = pd.shipping_method.calculator.dup