LINQ与设子查询 [英] Linq with subquery with Let

问题描述

需要用LINQ查询一些帮助。我试图从查询得到的是所有的广告获取最新的相关日志与LogType.IsStatus ==真有LogType.Name的确认或续期的列表。需要明确的是，一个广告有许多日志和日志每有一个LOGTYPE。到目前为止，我有以下的，但它给我的LastOrDefault错误System.NotSupportedException。任何想法？

  VAR广告=（来自于database.Adverts
                      让lastlog的=（从升的a.Logs
                                     其中，l.LogType.IsStatus == TRUE
                                     排序依据l.Created_at
                                     选择L）.LastOrDefault（）
                      其中（lastlog的！= NULL）
                            ＆功放;＆安培;
                            （lastLog.LogType.Name ==确认|| lastLog.LogType.Name ==续期）
                            排序依据a.Created_at降
                      选择一个）.ToList（）;
 

由于提前，

周杰伦

编辑：
新版本中，如果任何人有在今后类似的问题。的解决办法是首先编译的所有项目的列表，则子查询中的两个步骤，而不是在一气呵成列表

  VAR广告=（从在database.Adverts.ToList（）
               让lastStatusLog = a.Logs.Where（L =＆GT; l.LogType.IsStatus ==真）.OrderByDescending（L = GT; l.Created_at）.FirstOrDefault（）
               其中，lastStatusLog！= NULL
               ＆功放;＆安培;
               （lastStatusLog.LogType.Name ==证实|| lastStatusLog.LogType.Name ==重申）
               排序依据a.Created_at下降
               选择一个）.ToList（）;
 

解决方案

LastOrDefault（）在LINQ是不支持实体（见的这里）。

您可以变通的作法是由条款变更为了您的让子查询到按 降序，然后用 FirstOrDefault（）代替。

  VAR广告=（来自于database.Adverts
               让lastlog的=（从升的a.Logs
                              其中，l.LogType.IsStatus == TRUE
                              排序依据l.Created_at降
                              选择L）.FirstOrDefault（）
               其中（lastlog的！= NULL）
                     ＆功放;＆安培;
                     （lastLog.LogType.Name ==确认|| lastLog.LogType.Name ==续期）
                      排序依据a.Created_at降
               选择一个）.ToList（）;
 

Need some assistance with a Linq query. What I'm trying to get from the query is a list of all Adverts where the most recent associated Log with a LogType.IsStatus == true has a LogType.Name of either Confirmed or Renewed. To be clear, an Advert has many Logs and each Log has one LogType. So far I have the following, but its giving me an error System.NotSupportedException on the LastOrDefault. Any Ideas?

var adverts = (from a in database.Adverts
                      let lastLog = (from l in a.Logs
                                     where l.LogType.IsStatus == true
                                     orderby l.Created_at
                                     select l).LastOrDefault()
                      where (lastLog != null)
                            &&
                            (lastLog.LogType.Name == "Confirmed" || lastLog.LogType.Name == "Renewed")
                            orderby a.Created_at descending
                      select a).ToList();

Thanks in advance,

Jay

EDIT: New version, in case anyone has a similar problem in the future. Solution was to first compile a list of all of the items, then subquery the list in two steps rather than in one go.

var adverts = (from a in database.Adverts.ToList()
               let lastStatusLog = a.Logs.Where(l => l.LogType.IsStatus == true).OrderByDescending(l => l.Created_at).FirstOrDefault()
               where lastStatusLog != null
               &&
               (lastStatusLog.LogType.Name == "Confirmed" || lastStatusLog.LogType.Name == "Renewed")
               orderby a.Created_at descending
               select a).ToList();

解决方案

LastOrDefault() is not supported in LINQ to Entities (see here).

You can work around this by changing the order by clause in your let subquery to an order by descending and then use FirstOrDefault() instead.

var adverts = (from a in database.Adverts
               let lastLog = (from l in a.Logs
                              where l.LogType.IsStatus == true
                              orderby l.Created_at descending
                              select l).FirstOrDefault()
               where (lastLog != null)
                     &&
                     (lastLog.LogType.Name == "Confirmed" || lastLog.LogType.Name == "Renewed")
                      orderby a.Created_at descending
               select a).ToList();

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