Question

我有以下Linq代码：

    var allRequests = model.GetAllRequests();

    var unsatisifedRequests = (from request in allRequests
                               where request.Satisfied == false
                               select request)
                               .OrderBy(r => r.RequestedOn)
                               .GroupBy(r => r.RequestedCountryId);

之后，我对unsatifiedRequests进行了foreach，为每个构建了一个新的TOARequestListSummary对象。这意味着如果我从查询中“返回”了4个项目，它将对数据库进行4次调用，每次循环获取单个行一次。

这似乎是使用Linq的错误方法，所以我尝试将此查询转换为使用投影直接返回TOARequestListSummary对象的查询，我想出了：

var summaries = (from request in allRequests
                             where request.Satisfied == false
                             group request by request.RequestedCountryId into requestGroups
                             select new TOARequestListSummary
                             {
                                 CountryName = requestGroups.First().RequestedCountry.Description,
                                 RequestCount = requestGroups.Count(),
                                 FirstRequested = requestGroups.First().RequestedOn
                             });

但是当我运行它时，我得到以下异常：

Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.

我知道Linq等同于EXISTS是包含的，但我不知道如何将它产生到查询中。

Answer 1

这应该适合你：

var summaries = (from request in allRequests
                 where request.Satisfied == false
                 group request by request.RequestedCountry into g
                 select new TOARequestListSummary
                 {
                     CountryName = g.Key.Description,
                     RequestCount = g.Count(),
                     FirstRequested = g.Min(i => i.RequestedOn)
                 });

在此查询的原始版本（您发布的第二个版本）中，您论坛的密钥为RequestedCountryId。虽然这在技术上会对其进行分组，但实际上您希望使用关联的对象。通过这种方式，您可以轻松访问所需的属性，而无需担心抓取第一个项目。

Answer 2

很抱歉，这是一个答案，而不是对Ryan的答案的额外评论，但它太长了不适合......

这很奇怪。在LinqPad中，以下是一种享受：

from request in TOARequests
where request.Satisfied == false
&& request.Active == true
orderby request.RequestedOn
group request by request.RequestedCountry into g
select new 
{
    CountryName = g.Key.Description,
    RequestCount = g.Count(),
    FirstRequested = g.First().RequestedOn
}

但是以下内容在C＃

中引发相同的转换异常

var summaries = (from request in context.Repository<TOARequest>()
                             where request.Satisfied == false
                             && request.Active == true
                             orderby request.RequestedOn
                             group request by request.RequestedCountry into g
                             select new
                             {
                                 CountryName = g.Key.Description,
                                 RequestCount = g.Count(),
                                 FirstRequested = g.First().RequestedOn
                             }).ToList();

我可以看到ToList（）的唯一区别，但是当我尝试枚举列表时，即使没有它，它也会抛出异常。

如何在Linq中的选择查询中使用Count和Group？

2 个答案: