Return a json dictionary representing this model.

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