LINQ to DataSet 多表左连接

Question

有很多地方展示了如何使用 LINQ 左连接两个表，但我无法在连接中添加更多数据（我有 15 个表）。我以为我把模式弄坏了，但我得到了一个错误。顺便说一句，这是针对强类型数据集的，尽管我不怀疑这很重要。虽然我实际上有更多的表要添加到联接中，但我试图让它在最初使用更少的联接时工作（星号是发生错误的地方）：

var results = 
    from e in DataSetHelper.ds.Employee
    join es in DataSetHelper.ds.EmployeeSkill on e.EmployeeId equals es.EmployeeId    into esGroup from esItem in esGroup.DefaultIfEmpty()
    join s in DataSetHelper.ds.Skill on **es.SkillId** equals s.SkillId                   into skillGroup from skillItem in skillGroup.DefaultIfEmpty()
    join er in DataSetHelper.ds.EmployeeRole on e.EmployeeId equals er.EmployeeId     into erGroup from erItem in erGroup.DefaultIfEmpty()
    join r in DataSetHelper.ds.Role on **er.RoleId** equals r.RoleId                      into rGroup from rItem in rGroup.DefaultIfEmpty()

我遇到了两个错误（同样的事情，但在不同的连接上）。它们位于查询的第 3 行和第 5 行。

对于es.SkillId,，错误是The name 'es' does not exist in the current context.

对于er.RoleId,，错误是The name 'er' does not exist in the current context.

再次，我需要将此模式用于另外十个连接，因此我希望该模式不会随着我的使用而增加复杂性。

Answer 1

这行得通。我还必须考虑连接列上出现空值的可能性。为此，我使用了空条件运算符。不幸的是，使用内存数据集与使用 SQL Server 一样慢（而且我什至没有所有连接或所有 where 子句测试到位）。看来我需要使用不同的方法。

var results = from e in _data.ds.Employee
              join es in _data.ds.EmployeeSkill on e.EmployeeId equals es.EmployeeId        into esGroup from esItem in esGroup.DefaultIfEmpty()
              join s in _data.ds.Skill on esItem?.SkillId equals s.SkillId                  into sGroup from skillItem in sGroup.DefaultIfEmpty()
              join er in _data.ds.EmployeeRole on e.EmployeeId equals er.EmployeeId         into erGroup from erItem in erGroup.DefaultIfEmpty()
              join r in _data.ds.Role on erItem?.RoleId equals r.RoleId                     into rGroup from rItem in rGroup.DefaultIfEmpty()
              join et in _data.ds.EmployeeTechnology on e.EmployeeId equals et.EmployeeId   into etGroup from etItem in etGroup.DefaultIfEmpty()
              join t in _data.ds.Technology on etItem?.TechnologyId equals t.TechnologyId   into tGroup from tItem in etGroup.DefaultIfEmpty()
              where
                e.FirstName.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                e.LastName.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                e.RMMarket.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                !e.IsSummaryNull() && e.Summary.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0
              select new SearchResult
              {
                  EmployeeId = e.EmployeeId,
                  Name = e.FirstName + " " + e.LastName,
                  Title = e.Title,
                  ImageUrl = e.IsImageUrlNull() ? string.Empty : e.ImageUrl,
                  Market = e.RMMarket,
                  Group = e.Group,
                  Summary = e.IsSummaryNull() ? string.Empty : e.Summary.Substring(1, e.Summary.Length < summaryLength ? e.Summary.Length - 1 : summaryLength),
                  AdUserName = e.AdUserName
              };

Answer 2

试试这个

var results = DataSetHelper.ds.Employee
.Join(DataSetHelper.ds.EmployeeSkill, e => e.EmployeeId, es => es.EmployeeId, (e, es) => new { e, es })
.Join(DataSetHelper.ds.Skill, esGroup  => esGroup.es.SkillId, s => s.SkillId, (esGroup, s) => new { esGroup.e, esGroup.es, s })
.Join(....

我已经展示了直到第一个多对多关系。一切都会有点长，但我希望你能处理好。