code: def document(self, document_id=None):
          
          if document_id is None:
              document_id = _auto_id()
  
          child_path = self._path + (document_id,)
          return self._client.document(*child_path)
docs: Create a sub-document underneath the current collection.
  
          Args:
              document_id (Optional[str]): The document identifier
                  within the current collection. If not provided, will default
                  to a random 20 character string composed of digits,
                  uppercase and lowercase and letters.
  
          Returns:
              ~.firestore_v1beta1.document.DocumentReference: The child
              document.