Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'key_as_string') and self.key_as_string is not None:
            _dict['key_as_string'] = datetime_to_string(self.key_as_string)
        if hasattr(self, 'key') and self.key is not None:
            _dict['key'] = self.key
        if hasattr(self,
                   'matching_results') and self.matching_results is not None:
            _dict['matching_results'] = self.matching_results
        if hasattr(self, 'event_rate') and self.event_rate is not None:
            _dict['event_rate'] = self.event_rate
        return _dict