如何在 LINQ to entities 中执行带有附加过滤的左连接？

Question

我有好几个table，主要的一个叫DefectRecord，其他的叫DefectArea，DefectLevel...等等还有一个叫DefectAttachment。这个问题是关于将 DefectRecord 与其他 table 加入以获得一个 ViewModel 以供进一步使用。我面临的困难部分是 DefectAttachment table.

DefectRecord 与 DefectAttachment 具有一对多关系。虽然一个缺陷记录可能根本没有附件，但可能有多个附件。

从逻辑上讲，我尝试在 DefectRecord 和 DefectAttachment 之间执行左连接，但还有一个要求：

如果有多个附件，select只有最旧的（即最旧的最旧的 CreatedDate 字段值）

我被这个要求困住了，我如何使用 LINQ-to-Entities 执行此操作？下面是我现在的代码：

 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();

*Query 变量是 IQueryable 对象，它获取相应 table 的完整列表。

Answer 1

按 Code 和 FilterName 对结果进行分组，然后对于内容采用组中日期最早的项目的内容

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，您可以：

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

如何在 LINQ to entities 中执行带有附加过滤的左连接？

How to perform a left join with an additional filtering in LINQ to entities?

c#

linq

linq-to-entities

entity-framework