public async Task UpdateLessonAsync()
        {
            //Arrange
            Theme theme = new Theme
            {
                Name = "ExampleName",
                Id   = 5
            };

            Mentor mentor = new Mentor
            {
                Id = 2
            };

            StudentGroup studentGroup = new StudentGroup
            {
                Id = 3
            };

            List <VisitDto> visitsDto = new List <VisitDto>()
            {
            };

            var foundLesson = new Lesson()
            {
                Id             = 7,
                MentorId       = 2,
                StudentGroupId = 3,
                ThemeId        = 5,
                Mentor         = mentor,
                StudentGroup   = studentGroup,
                Theme          = theme,
                Visits         = { }
            };

            var updateLessonDto = new UpdateLessonDto
            {
                ThemeName    = null,
                LessonDate   = DateTime.Parse("2020-11-18T15:30:00.384Z"),
                LessonVisits = null
            };

            var foundLessonDto = new LessonDto()
            {
                Id             = 7,
                ThemeName      = "ExampleName",
                MentorId       = 2,
                StudentGroupId = 3,
                LessonDate     = DateTime.Parse("2020-11-18T15:00:00.384Z"),
                LessonVisits   = null
            };

            var updatedLesson = new LessonDto()
            {
                Id             = 7,
                ThemeName      = "ExampleName",
                MentorId       = 2,
                StudentGroupId = 3,
                LessonDate     = DateTime.Parse("2020-11-18T15:30:00.384Z"),
                LessonVisits   = visitsDto
            };

            _unitOfWorkMock.Setup(x => x.LessonRepository.GetByIdAsync(7))
            .ReturnsAsync(foundLesson);

            var lessonService = new LessonService(
                _unitOfWorkMock.Object,
                _mapper
                );

            //Act
            var result = await lessonService.UpdateLessonAsync(7, updateLessonDto);

            //Assert
            Assert.NotNull(result);

            Assert.Equal(updatedLesson.Id, result.Id);
            Assert.Equal(updatedLesson.LessonDate, result.LessonDate);
            Assert.Equal(updatedLesson.LessonVisits, result.LessonVisits);
            Assert.Equal(updatedLesson.MentorId, result.MentorId);
            Assert.Equal(updatedLesson.StudentGroupId, result.StudentGroupId);
            Assert.Equal(updatedLesson.ThemeName, result.ThemeName);
        }