Select Max() with GroupBy, using linq method syntax, EF - asp.net

Question

I have a bunch of classes with some data:
public class Teacher
{
public int Id { get; set; }
public string Name { get; set; }
public DateTime bDate { get; set; }
//One-to-one with course
public int CourseId { get; set; }
public virtual Course Course { get; set; }
}
public class Course
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection <Student_Course> Student_Courses { get; set; }
public int TeacherId { get; set; }
public virtual Teacher Teacher { get; set; }
}
public class Grade
{
public int Id { get; set; }
public int Mark { get; set; }
//one-to-many with Student_Course
public int Student_CourseId { get; set; }
public Student_Course Student_Course { get; set; }
}
public class Student_Course
{
public int Id { get; set; }
//many-to-many with student
public int StudentId { get; set; }
public virtual Student Student { get; set; }
//many-to-many with course
public int CourseId { get; set; }
public virtual Course Course { get; set; }
}
public class Student
{
public int Id { get; set; }
public string Name { get; set; }
public DateTime bDate { get; set; }
//one-to-many with student_course
public virtual ICollection <Student_Course> Student_Courses { get; set; }
}
Now i trying to write a query - for everyone teacher(name) needs to output his best student(name) by student mark, and the best mark, from class Grades, my code:
public IActionResult Task9()
{
var task9 = Db.Grades.GroupBy(gr => gr.Student_Course.Course.Teacher.Name).Select(x => new Task9
{
NameOfSt = x.Select(st => st.Student_Course.Student.Name).FirstOrDefault(),//Needs to correct this line
NameOfTeac = x.Select(r => r.Student_Course.Course.Teacher.Name).FirstOrDefault(),
BestMark = x.Max(gr => gr.Mark)
//NameOfSt = Db.Students.FirstOrDefault(st => st.Id ==x.FirstOrDefault().Student_Course.StudentId).Name
});
return View(task9);
}
So that code output correct teacher name and best mark on the course from students. But the student name is actually wrong. How can i fix that? Thanks for any help.
Teac
BestMark
NameStud

Answer 1

You could use OrderByDescending by Mark before select student name:
var task9 = Db.Grades.GroupBy(gr => gr.Student_Course.Course.Teacher.Name).Select(x => new Task9
{
NameOfSt = x.OrderByDescending(st => st.Mark).Select(st => st.Student_Course.Student.Name).FirstOrDefault(),//Needs to correct this line
NameOfTeac = x.Select(r => r.Student_Course.Course.Teacher.Name).FirstOrDefault(),
BestMark = x.Max(gr => gr.Mark)
});

Answer 2

Within the GroupBy you need to use the value of the grouping (in this case x) to select the Grade object with the highest Grade. In the code below, I get the bestGrade object by ordering by the Mark in descending order and then taking the first entry and then I take the student, name and best mark from that object:
public IActionResult Task9()
{
var task9 = Db.Grades.GroupBy(gr => gr.Student_Course.Course.Teacher.Name).Select(x =>
{
var bestGrade = x.OrderByDescending(y => y.Mark).First();
return new Task9
{
NameOfSt = bestGrade.Student_Course.Student.Name,//Needs to correct this line
NameOfTeac = bestGrade.Student_Course.Course.Teacher.Name,
BestMark = bestGrade.Mark
};
});
return View(task9);
}