Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self,
                   'matching_results') and self.matching_results is not None:
            _dict['matching_results'] = self.matching_results
        if hasattr(self, 'hits') and self.hits is not None:
            _dict['hits'] = [x._to_dict() for x in self.hits]
        return _dict