public async Task <ActionResult> Enroll(int id)
        {
            await _courseService.AddStudentAsync(id, User.Identity.GetUserId());

            if (User.IsInRole(Constants.RoleInstructor))
            {
                return(RedirectToRoute("InstructorDetails", new { id }));
            }
            else
            {
                return(RedirectToRoute("StudentDetails", new { id }));
            }
        }