docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'environment_id') and self.environment_id is not None:
              _dict['environment_id'] = self.environment_id
          if hasattr(self, 'customer_id') and self.customer_id is not None:
              _dict['customer_id'] = self.customer_id
          if hasattr(self, 'document_type') and self.document_type is not None:
              _dict['document_type'] = self.document_type
          if hasattr(self, 'natural_language_query'
                    ) and self.natural_language_query is not None:
              _dict['natural_language_query'] = self.natural_language_query
          if hasattr(self,
                     'document_results') and self.document_results is not None:
              _dict['document_results'] = self.document_results._to_dict()
          if hasattr(self,
                     'created_timestamp') and self.created_timestamp is not None:
              _dict['created_timestamp'] = datetime_to_string(
                  self.created_timestamp)
          if hasattr(self,
                     'client_timestamp') and self.client_timestamp is not None:
              _dict['client_timestamp'] = datetime_to_string(
                  self.client_timestamp)
          if hasattr(self, 'query_id') and self.query_id is not None:
              _dict['query_id'] = self.query_id
          if hasattr(self, 'session_token') and self.session_token is not None:
              _dict['session_token'] = self.session_token
          if hasattr(self, 'collection_id') and self.collection_id is not None:
              _dict['collection_id'] = self.collection_id
          if hasattr(self, 'display_rank') and self.display_rank is not None:
              _dict['display_rank'] = self.display_rank
          if hasattr(self, 'document_id') and self.document_id is not None:
              _dict['document_id'] = self.document_id
          if hasattr(self, 'event_type') and self.event_type is not None:
              _dict['event_type'] = self.event_type
          if hasattr(self, 'result_type') and self.result_type is not None:
              _dict['result_type'] = self.result_type
          return _dict