book.cached_author = ', '.join(
TagRelation.objects.filter(content_type__model='book', object_id=book.id, tag__category='author')
.values_list('tag__name', flat=True))
book.cached_author = ', '.join(
TagRelation.objects.filter(content_type__model='book', object_id=book.id, tag__category='author')
.values_list('tag__name', flat=True))