public IActionResult Index()
        {
            var students = _studentService.GetAll()
                           .Select(student => new StudentsListItem {
                Id = student.Id, Name = student.Name
            });
            var model = new CourseStudentIndexModel
            {
                Students = students
            };

            return(View(model));
        }
        public IActionResult Detail(int id)
        {
            var course   = _courseService.GetById(id);
            var students = course.Students
                           .Select(s => new StudentsListItem
            {
                Id   = s.Id,
                Name = s.Name
            });

            var model = new CourseStudentIndexModel
            {
                Students = students
            };

            return(View(model));
        }