public OrderHeader UpsertOrderItem(int orderHeaderId, int stockItemId, int quantity)
        {
            StockItem   item      = stockRepo.GetStockItem(stockItemId);
            OrderHeader order     = orderRepo.GetOrderHeader(orderHeaderId);
            var         orderItem = order.AddOrderItem(stockItemId, item.Price, item.Name, quantity);

            orderRepo.UpsertOrderItem(orderItem);
            return(order);
        }