public async Task get_single_async_should_not_invoke_set_repository_get_single_async_by_id_when_set_does_not_exist()
        {
            var setService = new SetService(_setRepositoryMock.Object, _mapperMock.Object);
            var book       = new Book("Lśnienie", "opis", "image.png", "Stephen King", "Prószyński i S-ka", 20);
            var set        = new Set(book, true);

            await setService.GetSingleAsync(set.Id);

            _setRepositoryMock.Setup(x => x.GetSingleAsync(set.Id))
            .ReturnsAsync(() => null);

            _setRepositoryMock.Verify(x => x.GetSingleAsync(It.IsAny <Guid>()), Times.Once);
        }