inventory_transaction.rb

class InventoryTransaction < ActiveRecord::Base
  
  belongs_to :inventory_adjustable, :polymorphic => true
  belongs_to :inventory
  
  after_create :adjust_inventory
  
  def adjust_inventory
    puts "current inventory: #{inventory.quantity_on_hand}"
    inventory.quantity_on_hand += adjustment
    inventory.save
  end
  
  def readjust_inventory(new_adjustment, new_description)
    inventory.quantity_on_hand += (adjustment * -1)
    
    self.adjustment = new_adjustment
    self.description = new_description
    self.save
    
    adjust_inventory
  end
end

shipment_item.rb

class ShipmentItem < ActiveRecord::Base
  belongs_to :shipment
  belongs_to :sales_order_item
  has_one :inventory_transaction, :as => :inventory_adjustable
  
  named_scope :shipped, :include => :shipment, :conditions => { 'shipments.state' => :shipped }
  
  # callback
  after_save :adjust_inventory
  
  def product
    @product ||= sales_order_item.product
  end
  
  def adjust_inventory
    if inventory_transaction
      inventory_transaction.readjust_inventory(self.quantity * -1, "Updated: #{quantity} for Shipment ##{self.shipment.id}")
      inventory_transaction.adjust_inventory
    else
      inventory_transaction = InventoryTransaction.new({
                                :adjustment => (self.quantity * -1),
                                :inventory_id => product.inventory_for_warehouse(shipment.warehouse).id,
                                :description => "#{quantity} for Shipment ##{self.shipment.id}"})
      inventory_transaction.save
    end
  end
  
end

shipment_item_test.rb

require 'test_helper'

class ShipmentItemTest < ActiveSupport::TestCase
  
  should_belong_to :shipment
  should_belong_to :sales_order_item
  should_have_one :inventory_transaction
  
  context "A ShipmentItem" do
    
    should "reduce the quantity on hand for it's product" do
      
      @shipment_item = Factory.build(:shipment_item)
      @inventory = Factory(:inventory, :product_id => @shipment_item.product.id, :warehouse_id => @shipment_item.shipment.warehouse_id)
      
      assert_difference '@inventory.quantity_on_hand', (@shipment_item.quantity * -1) do
        @shipment_item.save
        @inventory.reload
      end
    end
    
    should "when updated, re-adjust it's inventory adjustment to reflect the new quantity" do
      @shipment_item = Factory.build(:shipment_item)
      @inventory = Factory(:inventory, :product_id => @shipment_item.product.id, :warehouse_id => @shipment_item.shipment.warehouse_id)
      
      assert_difference '@inventory.quantity_on_hand', (@shipment_item.quantity * -1) do
        @shipment_item.save
        @inventory.reload
      end
      
      old_quantity = @shipment_item.quantity
      @shipment_item.quantity = 30
      assert_difference '@inventory.quantity_on_hand', (@shipment_item.quantity * -1) - (old_quantity * -1) do
        @shipment_item.save
        @inventory.reload
      end
    end
    
  end
  
end