需要检查数据库，以便查看是否购买了商品 [英] Need check the database so see if an item was purchase

问题描述

我正在尝试一个if/else.我需要首先检查数据库order_details表以查看数量是否为零.如果大于零，请转到提交页面，否则请转到主页.


这就是我所拥有的.

I am trying to do an if/else. I need to first check the database order_details table to see if the quantity is zero. If greater than zero go to submission page else go to home page.


This is what I have.

public ActionResult Patients()
       {
            //query database for userId RoleId and Purchase product.
            var query = (from u in db.aspnet_Users
                         where u.UserName == User.Identity.Name
                         join d in db.Doctors on u.UserId equals d.UserId
                         join o in db.OrderDetails on d.DoctorsId equals o.DoctorsId
                         select o).FirstOrDefault().Quantity;
            //check against the quantity table for valid purchase.
            if (query > 0)

            {
                 return View();
            }
            else
            {
                 return RedirectToAction("Index", "Home");
            }
       }

如果订单明细"数量列为零，则会崩溃.
我登录时使用的任何一个在我的数据库的数量"列中都有一个数字即可.

我希望我能解释这一权利.要在用户单击选项卡时转到患者提交页面，需要查询数据库以查看是否有购买服务.

If the Order Details quantity column is zero it crashes.
Any one I Log on with that has a number in the quantity column in my database it works.

I hope I am explaining this right. To go to the patients submit page when a user clicks on the tab it need to query the database to check to see if purchase services.

推荐答案

<如果没有项目，则c0>方法返回null.

在使用它之前，您必须检查从FirstOrDefault方法返回的值.类似于以下内容:

You have to check the value that is returned from from the FirstOrDefault method, before using it. Something like the following:

var orderDetail = (from u in db.aspnet_Users
            where u.UserName == User.Identity.Name
            join d in db.Doctors on u.UserId equals d.UserId
            join o in db.OrderDetails on d.DoctorsId equals o.DoctorsId
            select o).FirstOrDefault();

var query = (orderDetail != null) ? orderDetail.Quantity : 0;

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