+ @classmethod
+ def tagged_top_level(cls, tags):
+ """ Returns top-level books tagged with `tags'.
+
+ It only returns those books which don't have ancestors which are
+ also tagged with those tags.
+
+ """
+ # get relevant books and their tags
+ objects = cls.tagged.with_all(tags)
+ # eliminate descendants
+ l_tags = Tag.objects.filter(category='book', slug__in=[book.book_tag_slug() for book in objects])
+ descendants_keys = [book.pk for book in cls.tagged.with_any(l_tags)]
+ if descendants_keys:
+ objects = objects.exclude(pk__in=descendants_keys)
+
+ return objects
+