c# - 如何制定 GroupJoin() 声明

Question

我有一个看起来(简化)如下的数据库:

var reports = new[] {
    new { ReportId = 1, Title = "Report A" },
    new { ReportId = 2, Title = "Report B" },
};

var testCases = new[] {
    new { TestId = 1, Title = "Test A" },
    new { TestId = 2, Title = "Test B" },
    new { TestId = 3, Title = "Test C" },
    new { TestId = 4, Title = "Test D" },
    new { TestId = 5, Title = "Test E" },
};

var testRuns = new[] {
    new { TestId = 1, ReportId = 1 },
    new { TestId = 2, ReportId = 1 },
    new { TestId = 1, ReportId = 2 },
    new { TestId = 2, ReportId = 2 },
    new { TestId = 3, ReportId = 2 },
    new { TestId = 4, ReportId = 2 },
};

因此，我想获得一个 testCases 列表，并与相应的 reports 分组，即:

Test A => [Report A, Report B]
Test B => [Report A, Report B]
Test C => [Report B]
Test D => [Report B]
Test E => []

我不确定如何在 SQL 或 LINQ 中表达这一点。我想我需要一个 group join 或一个 left outer join 或者不管它叫什么，但我还不知道正确的语法。我试过这样的东西，但仍然遗漏了一部分:

var result = tests.GroupJoin(reports, t => t.TestId, ???, (t, rs) => new { Test = t, Reports = rs });

或者可能有一种完全不同的方式来制定该查询。

编辑:并非 testCases 中的每个条目都在 testRuns 中被引用。

Answer 1

如果您可以不用 GroupJoin 也能生活，您可以使用 2 个简单的连接和一个 GroupBy，如下所示:

var temp = from c in testCases
           join ru in testRuns
           on c.TestId equals ru.TestId into left
           from l in left
           join re in reports 
           on l.ReportId equals re.ReportId into foo
           from f in foo
           select new {
                Test = c.Title,
                Report = f
           };
//Dump only in LinqPad!            
temp.GroupBy(x => x.Test).Dump();

/编辑:如果你还想要空结果，你需要使用DefaultIfEmpty():

public class Report
{
    public int ReportId { get; set; }
    public string Title { get; set; }
}

public class TestCase
{
    public int TestId { get; set; }
    public string Title { get; set; }
}

public class TestRun
{
    public int TestId { get; set; }
    public int ReportId { get; set; }
}
var temp = from c in testCases
           join ru in testRuns
           on c.TestId equals ru.TestId into left
           from l in left.DefaultIfEmpty(new TestRun())
           join re in reports 
           on l.ReportId equals re.ReportId into foo
           from f in foo.DefaultIfEmpty()
           select new {
                Test = c.Title,
                Report = f 
           };

temp.Dump();

• 演示代码:http://share.linqpad.net/t3osbj.linq
• 使用 DefaultIfempty 的强类型演示:http://share.linqpad.net/fsov4p.linq
• 使用 DefaultIfEmpty 的松散类型示例:http://share.linqpad.net/7k2lq4.linq