Return a json dictionary representing this model.

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