private readonly ApplicationDbContext _context;
        public HomeController(ApplicationDbContext context)
        { 
            _context = context;
        }

        public IActionResult Index()
        {
            //add grades
            var grades = new List<Grade>()
            {
                new Grade(){GradeName = "Grade 1" },
                new Grade(){GradeName = "Grade 2" },
            };
            _context.Grades.AddRange(grades);
            _context.SaveChanges();

            //find the existing grade
            var grade1 = _context.Grades.FirstOrDefault(c => c.GradeName == "Grade 1");
            //if the grade is not exist, create a new grade

            //add student with grade.
            var student = new List<Student>() { 
                new Student() { Name = "Tom", Grade = grade1 },
                 new Student() { Name = "John", Grade = grade1 },
            };
            _context.Students.AddRange(student);
            _context.SaveChanges();

            //query grades and the relates students
            var queryresult = _context.Grades.Include(c => c.Students).ToList();
 
            return View();
        }