Populate the line item unit value, when line_item created and update old data in migration

app/models/spree/order_decorator.rb

@@ -127,6 +127,7 @@ Spree::Order.class_eval do
     else
       current_item = Spree::LineItem.new(:quantity => quantity, max_quantity: max_quantity)
       current_item.variant = variant
+      current_item.unit_value = variant.unit_value
       if currency
         current_item.currency = currency unless currency.nil?
         current_item.price    = variant.price_in(currency).amount

db/migrate/20150424151117_populate_line_item_unit_value.rb

@@ -0,0 +1,9 @@
+class PopulateLineItemUnitValue < ActiveRecord::Migration
+  def up
+    execute "UPDATE spree_line_items SET unit_value = spree_variants.unit_value FROM spree_variants WHERE spree_line_items.variant_id = spree_variants.id"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end