如何在LINQ中使用額外的過濾功能執行左連接？

Question

我有幾張桌子，主要是DefectRecord，其他叫做DefectArea,DefectLevel ......等，還有一個叫DefectAttachment。而這個問題是關於加入DefectRecord與其他表來獲得ViewModel進一步使用。我面臨的難題是關於DefectAttachment表格。

DefectRecord與DefectAttachment有一對多的關係。儘管一個缺陷記錄可能沒有附件，但可能有多個附件。

按道理我試圖執行一個左連接中DefectRecord & DefectAttachment，但有一個更requiredment：

• 如果有多個附件，只選擇最老的（即 一個與最古老的CreatedDate場值）

我被困在這個要求，我怎麼能用LINQ-to-Entities執行此操作？下面是什麼，我現在有代碼：

var ret = (from dr in defectRecordQuery 
      join ft in filterQuery on dr.FilterID equals ft.FilterID 
      join l in levelQuery on dr.LevelID equals l.LevelID 
      join a in attachmentQuery on dr.DefectRecordID equals a.DefectRecordID into drd 
      from g in drd.DefaultIfEmpty() 
      select new DefectRecordViewModel 
      { 
       DefectRecordCode = dr.Code, 
       DefectAttachmentContent = g == null ? null : g.FileContent, 
       LookupFilterName = ft.FilterName, 
      }).ToList(); 

的*查詢變量是IQueryable對象，獲得相應的表的完整列表。

Answer 1

組的結果由Code和FilterName，然後對內容採取具有最早的日期

var ret = (from dr in defectRecordQuery 
      join ft in filterQuery on dr.FilterID equals ft.FilterID 
      join l in levelQuery on dr.LevelID equals l.LevelID 

      join d in attachmentQuery on dr.DefectRecordID equals d.DefectRecordID into drd 
      from g in drd.DefaultIfEmpty() 
      group g by new { dr.Code, ft.FilterName } into gg 

      select new DefectRecordViewModel 
      { 
       DefectRecordCode = gg.Key.Code, 
       DefectAttachmentContent = gg.OrderByDescending(x => x.CreateDateTime).FirstOrDefault() == null? null: gg.OrderByDescending(x => x.CreateDateTime).FirstOrDefault().FileContent, 
       LookupFilterName = gg.Key.FilterName, 
      }).ToList(); 

如果使用C＃6.0或更高版本，那麼你可以做的組中的項目：

。

DefectAttachmentContent = gg.OrderByDescending(x => x.CreateDateTime) 
          .FirstOrDefault()?.FileContent,