text: from rest_framework.fields import Field
  
  from wagtail.api.v2.serializers import BaseSerializer
  from wagtail.api.v2.utils import get_full_url
  
  
  class DocumentDownloadUrlField(Field):
      """
      Serializes the "download_url" field for documents.
  
      Example:
      "download_url": "http://api.example.com/documents/1/my_document.pdf"
      """
      def get_attribute(self, instance):
          return instance
  
      def to_representation(self, document):
          return get_full_url(self.context['request'], document.url)
  
  
  class DocumentSerializer(BaseSerializer):
      download_url = DocumentDownloadUrlField(read_only=True)
  