public async void StudentId_Create_Valid_Reference()
        {
            Mock <ILessonXStudentRepository> lessonXStudentRepository = new Mock <ILessonXStudentRepository>();

            lessonXStudentRepository.Setup(x => x.GetStudent(It.IsAny <int>())).Returns(Task.FromResult <Student>(new Student()));

            var validator = new ApiLessonXStudentRequestModelValidator(lessonXStudentRepository.Object);
            await validator.ValidateCreateAsync(new ApiLessonXStudentRequestModel());

            validator.ShouldNotHaveValidationErrorFor(x => x.StudentId, 1);
        }
        public async void LessonId_Update_Valid_Reference()
        {
            Mock <ILessonXStudentRepository> lessonXStudentRepository = new Mock <ILessonXStudentRepository>();

            lessonXStudentRepository.Setup(x => x.GetLesson(It.IsAny <int>())).Returns(Task.FromResult <Lesson>(new Lesson()));

            var validator = new ApiLessonXStudentRequestModelValidator(lessonXStudentRepository.Object);
            await validator.ValidateUpdateAsync(default(int), new ApiLessonXStudentRequestModel());

            validator.ShouldNotHaveValidationErrorFor(x => x.LessonId, 1);
        }