Return a json dictionary representing this model.

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