About Nested Levels at Elasticsearch Index

from django.conf import settings
from django_elasticsearch_dsl.registries import registry
from django_elasticsearch_dsl import Document, fields
from .models import Book

@registry.register_document
class BookDocument(Document):
    authors = fields.NestedField(
        properties={
            "uuid": fields.KeywordField(),
            "first_name": fields.TextField(),
            "last_name": fields.TextField(),
            "photo": fields.ObjectField(
                properties={
                    "image_url": fields.TextField(),
                    "copyright": fields.TextField(),
                }
            ),
        }
    )

    class Index:
        name = f"{settings.PROJECT_NAME}_{settings.ENVIRONMENT}_books"
        settings = {
            "number_of_shards": 1,
            "number_of_replicas": 0,
        }

    class Django:
        model = Book

        fields = ["uuid", "title"]

        ignore_signals = False
        auto_refresh = True
        queryset_pagination = 5000

    def prepare_authors(self, instance):
        return [{
            "uuid": author.uuid,
            "first_name": author.first_name,
            "last_name": author.last_name,
            "photo": {
                "image_url": author.photo.image.url,
                "copyright": author.photo.copyright,
            }
        } for author in instance.authors.all()]