public ActionResult RemoveStudentFromGroup(RemoveStudentFromGroup model)
        {
            //complete operation
            GroupService service = new GroupService();
            service.RemoveUserFromGroup(db, model.StudentId);

            return RedirectToAction("Index","Group");
        }
        public ActionResult AddStudentToGroup(AddStudentToGroup model)
        {
            //complete operation
            GroupService service = new GroupService();
            service.AddUserToGroup(db, model.StudentId, model.GroupId, model.Date);

            return RedirectToAction("Index", "Group");
        }