You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fulfilment_changer.rb
    path of file: ./repos/spree/core/app/models/spree
    
    the code of the file until where you have to start completion: module Spree
  class FulfilmentChanger
    inclu
    
    def update_desired_shipment_inventory_units
      on_hand_unit = get_desired_shipment_inventory_unit(:on_hand)
      on_hand_unit.update(quantity: on_hand_unit.quantity + new_on_hand_quantity)

      new_backorder_quantity = current_on_hand_quantity - new_on_hand_quantity
      if new_backorder_quantity > 0
        backordered_unit = get_desired_shipment_inventory_unit(:backordered)
        backordered_unit.update(quantity: backordered_unit.quantity + new_backorder_quantity)
      end