"""
books_by_parent = {}
- books = cls.objects.order_by('parent_number', 'sort_key').only('title', 'parent', 'slug')
+ books = cls.objects.order_by('parent_number', 'sort_key').only('title', 'parent', 'slug', 'extra_info')
if book_filter:
books = books.filter(book_filter).distinct()