public ActionResult AddStudentToGroup(AddStudentToGroup model)
        {
            //complete operation
            GroupService service = new GroupService();
            service.AddUserToGroup(db, model.StudentId, model.GroupId, model.Date);

            return RedirectToAction("Index", "Group");
        }
        public ActionResult AddStudentToGroup(int id)
        {
            Student student = db.Students.Find(id);

            if (student == null)
            {
                return HttpNotFound();
            }

            ViewBag.GroupId = new SelectList(db.Groups, "Id", "Name");

            var model = new AddStudentToGroup()
            {
                StudentId = id,
                Date = DateTime.Now
            };

            return View(model);
        }