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);
}