Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'relation_types') and self.relation_types is not None:
            _dict['relation_types'] = self.relation_types._to_dict()
        if hasattr(self, 'entity_types') and self.entity_types is not None:
            _dict['entity_types'] = self.entity_types._to_dict()
        if hasattr(self, 'document_ids') and self.document_ids is not None:
            _dict['document_ids'] = self.document_ids
        return _dict