c# - 在 LINQ 中按子表分组

Question

我有学生表和科目表，我需要按科目对学生进行分组。我尝试了以下不显示 s.StudentSubjects.SubjectName 的内容。我怎样才能写 group by with child table 。

学生 -> StudentID |姓名
StudentSubjects -> 主题 ID |学号 |主题名称

        var list = from s in students
                   group s by s.StudentSubjects.? into g
                   select new StudentSubjectsCounts
                   {
                       Name = g.Key,
                       Count = g.Count(),                           
                   };

Answer 1

听起来你应该查询 StudentSubjects 而不是 Student:

var list = from ss in studentSubjects
           group ss by s.SubjectName into g
           select new StudentSubjectsCounts
           {
               Name = g.Key,
               Count = g.Count(),                           
           };

或者，从学生列表开始:

var list = students.SelectMany(s => s.StudentSubjects)
                   .GroupBy(ss => ss.SubjectName)
                   .Select(g => new StudentSubjectsCounts
                       {
                           Name = g.Key,
                           Count = g.Count(),                           
                       });