def _openapi_json(self):
        """Serve JSON spec file"""
        # We don't use Flask.jsonify here as it would sort the keys
        # alphabetically while we want to preserve the order.
        from pprint import pprint
        pprint(self.to_dict())
        return current_app.response_class(json.dumps(self.to_dict(), indent=4),
                                          mimetype='application/json')