public Achievement(Festival festival, Discipline discipline, Pupil pupil)
        {
            if (festival == null)
                throw new ArgumentNullException(nameof(festival));
            if (discipline == null)
                throw new ArgumentNullException(nameof(discipline));
            if (pupil == null)
                throw new ArgumentNullException(nameof(pupil));

            Festival = festival;
            Discipline = discipline;
            Pupil = pupil;
        }
        public static Achievement GetAchievement(SportsFestivalManagerContext connection, Festival festival, Discipline discipline, Pupil pupil)
        {
            var achievement = connection.Achievements.FirstOrDefault(x => x.FestivalId == festival.Id && x.DisciplineId == discipline.Id && x.PupilId == pupil.Id);

            if (achievement == null)
                connection.Achievements.Add(achievement = new Achievement(festival, discipline, pupil));

            return achievement;
        }