public async Task <IActionResult> CreateExam([FromBody] CreatingExamModel model)
        {
            if (!ModelState.IsValid)
            {
                return(BadRequest(ModelState));
            }

            // Todo: need to check all fields before entering a new one.
            // Todo: Exams with the same date, but with different rooms, are two separate exams
            var exam = examService.FindByTime(model.Date);

            if (exam.Result == null)
            {
                var examId = await examService.CreateNew(model);

                return(StatusCode(StatusCodes.Status201Created, examId));
            }

            return(Conflict());
        }