public async Task Exclude_DeleteReferenseTeacher()
        {
            // Arrange
            var         groupId    = new Guid(GroupId);
            GroupEntity testEntity = null;

            var teacher = new EmployeeEntity {
                Id = new Guid(TeacherId)
            };

            teacher.Groups.Add(groupId);

            var mockEmployee = new Mock <IEmployeeService>();
            var mockStudent  = new Mock <IStudentService>();

            var mockRepo = new Mock <IGroupRepository>();

            mockRepo.Setup(repo => repo.GetByIdAsync(groupId)).Returns(Task.FromResult(GetGroups().First()));
            mockRepo.Setup(repo => repo.UpdateAsync(groupId, It.IsAny <GroupEntity>()))
            .Callback((Guid id, GroupEntity entity) =>
            {
                testEntity = entity;
            });

            var mockUnitOfWork = new Mock <IUnitOfWork>();

            mockUnitOfWork.Setup(uow => uow.Groups).Returns(mockRepo.Object);

            var provider = new GroupProvider(mockUnitOfWork.Object, mockStudent.Object, mockEmployee.Object, CreateMapper());

            // Act
            await provider.Exclude(g => g.DeleteReferenseTeacher(), teacher);

            // Assert
            Assert.Equal(default, testEntity.TeacherId);