Question

我有几个表，主要的一个称为DefectRecord，其他表称为DefectArea，DefectLevel ...等，另一个称为DefectAttachment。这个问题是关于将DefectRecord与其他表连接以获取ViewModel以供进一步使用。我面临的困难是关于DefectAttachment表。

DefectRecord与DefectAttachment具有1对多的关系。虽然一个缺陷记录可能根本没有附件，但可能有多个附件。

逻辑上我尝试在DefectRecord＆amp;之间执行左连接。 DefectAttachment，但还有一项要求：

如果有多个附件，请仅选择最旧的附件（即一个具有最早CreatedDate字段值的

我坚持这个要求，如何使用LINQ-to-Entities 执行此操作？以下是我现在的代码：

 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，它获取相应表的完整列表。

Answer 1

按Code和FilterName对结果进行分组，然后将内容视为具有最早日期的组中项目的内容

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，则可以执行以下操作：

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

如何在LINQ实体中执行额外过滤的左连接？

1 个答案: