Return a json dictionary representing this model.

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