Replace shipping method lookup with relation

app/models/spree/line_item_decorator.rb

@@ -1,9 +1,7 @@
 Spree::LineItem.class_eval do
-  attr_accessible :max_quantity
+  belongs_to :shipping_method
 
-  def shipping_method
-    self.product.shipping_method_for_distributor(self.order.distributor)
-  end
+  attr_accessible :max_quantity
 
   def itemwise_shipping_cost
     order = OpenStruct.new :line_items => [self]

db/migrate/20120919013335_add_shipping_method_to_line_items.rb

@@ -0,0 +1,19 @@
+class AddShippingMethodToLineItems < ActiveRecord::Migration
+  def up
+    add_column :spree_line_items, :shipping_method_id, :integer
+
+    Spree::LineItem.all.each do |li|
+      begin
+        shipping_method = li.product.shipping_method_for_distributor(li.order.distributor)
+      rescue ArgumentError
+        shipping_method = Spree::ShippingMethod.find_by_name 'Producer Delivery'
+      end
+
+      li.update_attribute(:shipping_method_id, shipping_method.id)
+    end
+  end
+
+  def down
+    remove_column :spree_line_items, :shipping_method_id
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20120913054660) do
+ActiveRecord::Schema.define(:version => 20120919013335) do
 
   create_table "distributors", :force => true do |t|
     t.string   "name"
@@ -180,11 +180,12 @@ ActiveRecord::Schema.define(:version => 20120913054660) do
   create_table "spree_line_items", :force => true do |t|
     t.integer  "order_id"
     t.integer  "variant_id"
-    t.integer  "quantity",                                   :null => false
-    t.decimal  "price",        :precision => 8, :scale => 2, :null => false
-    t.datetime "created_at",                                 :null => false
-    t.datetime "updated_at",                                 :null => false
+    t.integer  "quantity",                                         :null => false
+    t.decimal  "price",              :precision => 8, :scale => 2, :null => false
+    t.datetime "created_at",                                       :null => false
+    t.datetime "updated_at",                                       :null => false
     t.integer  "max_quantity"
+    t.integer  "shipping_method_id"
   end
 
   add_index "spree_line_items", ["order_id"], :name => "index_line_items_on_order_id"