class AddSerialNumbertoServiceVehicles < ActiveRecord::DataMigration
  def up
    ServiceVehicle.all.each do |sv|
      unless sv.serial_numbers.first.nil?
        old_serial_number = sv.serial_numbers.first
        vin = old_serial_number.identification
        sv.serial_number = vin
        sv.save 
        old_serial_number.delete
      end
    end 
  end

  def down
    ServiceVehicle.all.each do |sv|
      old_serial_number = SerialNumber.new
      old_serial_number.identifiable_type = 'TransamAsset'
      old_serial_number.identifiable_id = sv.id 
      old_serial_number.identification = sv.serial_number
      old_serial_number.save 
    end 
  end
end
class AddSerialNumbertoServiceVehicles < ActiveRecord::DataMigration
  def up
    ServiceVehicle.all.each do |sv|
      old_serial_number = sv.serial_numbers.first
      vin = old_serial_number.identification
      sv.serial_number = vin
      sv.save 
      old_serial_number.delete
    end 
  end

  def down
    ServiceVehicle.all.each do |sv|
      old_serial_number = SerialNumber.new
      old_serial_number.identifiable_type = 'TransamAsset'
      old_serial_number.identifiable_id = sv.id 
      old_serial_number.identification = sv.serial_number
      old_serial_number.save 
    end 
  end
end
--- 
+++ 
@@ -1,11 +1,13 @@
 class AddSerialNumbertoServiceVehicles < ActiveRecord::DataMigration
   def up
     ServiceVehicle.all.each do |sv|
-      old_serial_number = sv.serial_numbers.first
-      vin = old_serial_number.identification
-      sv.serial_number = vin
-      sv.save 
-      old_serial_number.delete
+      unless sv.serial_numbers.first.nil?
+        old_serial_number = sv.serial_numbers.first
+        vin = old_serial_number.identification
+        sv.serial_number = vin
+        sv.save 
+        old_serial_number.delete
+      end
     end 
   end
 
Update Serial Number data migration
