我正在构建一个web应用程序，当用户登录并尝试从web购买某样东西时，此错误出现
SQL异常：当IDENTITY_INSERT设置为OFF时，无法为表'orderFactors'中的标识列插入显式值。

public IActionResult AddToCart(int itemId)
    {
        var product = _context.Products.Include(p => p.Item).SingleOrDefault(p => p.ItemId == itemId);
        if (product != null)
        {
            int userId = int.Parse(User.FindFirstValue(ClaimTypes.NameIdentifier).ToString());
            var order = _context.orderFactors.FirstOrDefault(o => o.UserId == userId && !o.IsFinally);
            if (order != null) 
            {
                var orderDetail = _context.orderDetails.FirstOrDefault(o => o.OrderId == order.OrderId && o.ProductId == product.Id);
                if (orderDetail != null) 
                {
                    orderDetail.Count += 1;
                }
                else
                {
                    _context.orderFactors.Add(order);
                    _context.orderDetails.Add(new OrderDetail()
                    {
                        OrderId = order.OrderId,
                        ProductId = product.Id,
                        price = product.Item.Price,
                        Count = 1
                    });
                }
            }
            else
            {
                order = new OrderFactor()
                {
                    IsFinally = false,
                    CreateDate= DateTime.Now,
                    UserId= userId

                };
                _context.orderFactors.Add(order);
                _context.SaveChanges();
                _context.orderDetails.Add(new OrderDetail()
                {
                    OrderId = order.OrderId,
                    ProductId = product.Id,
                    price = product.Item.Price,
                    Count = 1
                });
            }
            _context.SaveChanges();
        }
        return RedirectToAction("ShowCart");
    }