Question

我有这种类型的代码。在Material表中，它只有一个MaterialId。但是在Report表中，它可以是多个MaterialId行。（一个materialId可以在Report表中有多个行）。

但是在Material表中我没有任何时间戳。但是在Report中有一个列为TimeStamp，我想用它来获取最新版本。

我现在只需要使用ReportAdminModel的不同值。

但是我怎么能让它像那样工作呢？我尝试了Distinct（），但这不起作用。

list = (from material in db.Material
                        join reports in db.Report on material.MaterialId equals reports.MaterialId
                        select new ReportAdminModel
                        { MaterialId = material.MaterialId, MaterialStatus = material.ProcessApprovalStatus, FlowIndex = material.FlowIndex, ActualStartTime = reports.Timestamp })
                        .OrderByDescending(x => x.ActualStartTime).Take(item.NumberOfRows.Value).ToList();

Answer 1

如果我理解正确，请尝试：

list = (from material in db.Material
                join reports in (from rep in db.Report
                                 group rep by rep.MaterialId into grp
                                 let latestTimeStamp = grp.Max(o => o.Timestamp)
                                 select new
                                 {
                                     MaterialId = grp.Key,
                                     Timestamp = latestTimeStamp,
                                     //if you need any other field, just do something like :
                                     //SomeField = grp.Where(o => o.Timestamp == latestTimeStamp).Select(o => o.SomeField).FirstOrDefault();
                                 })    
                on material.MaterialId equals reports.MaterialId
                select new ReportAdminModel
                {MaterialId = material.MaterialId, 
                 MaterialStatus = material.ProcessApprovalStatus, 
                 FlowIndex = material.FlowIndex, 
                 ActualStartTime = reports.Timestamp, 
                }).OrderByDescending(x => x.ActualStartTime).ToList();

Answer 2

您可以group之前应用material.MaterialId, material.ProcessApprovalStatus, material.FlowIndex并检索每个组项的最后Timestamp个数据。

list = (from material in db.Material
        join reports in db.Report on material.MaterialId equals reports.MaterialId
        group new{material,reports} by new { material.MaterialId, material.ProcessApprovalStatus, material.FlowIndex } into materialGrouped
        select new ReportAdminModel
        { 
            MaterialId = materialGrouped.Key.MaterialId, 
            MaterialStatus = materialGrouped.Key.ProcessApprovalStatus, 
            FlowIndex = materialGrouped.Key.FlowIndex, 
            ActualStartTime = materialGrouped.Max(x => x.reports.TimeStamp)
        })
        .OrderByDescending(x => x.ActualStartTime).Take(item.NumberOfRows.Value).ToList();

linq中带有join子句的不同值

2 个答案: