Return a json dictionary representing this model.

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, 'session_token') and self.session_token is not None:
            _dict['session_token'] = self.session_token
        if hasattr(self,
                   'client_timestamp') and self.client_timestamp is not None:
            _dict['client_timestamp'] = datetime_to_string(
                self.client_timestamp)
        if hasattr(self, 'display_rank') and self.display_rank is not None:
            _dict['display_rank'] = self.display_rank
        if hasattr(self, 'collection_id') and self.collection_id is not None:
            _dict['collection_id'] = self.collection_id
        if hasattr(self, 'document_id') and self.document_id is not None:
            _dict['document_id'] = self.document_id
        if hasattr(self, 'query_id') and self.query_id is not None:
            _dict['query_id'] = self.query_id
        return _dict