public async Task <ActionResult <IEnumerable <SubjectDto> > > GetTimetableAsync(Guid studentId)
        {
            if (!_repository.StudentExists(studentId))
            {
                return(NotFound());
            }

            // get lessons
            // order lessons
            var student = await _repository.GetStudentAsync(studentId);

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

            var lessons = await _repository.GetLessonsForStudent(studentId);

            return(Ok(lessons));
        }