Return a json dictionary representing this model.

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