Return a json dictionary representing this model.

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