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);
}