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, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self, 'top_class') and self.top_class is not None:
            _dict['top_class'] = self.top_class
        if hasattr(self, 'classes') and self.classes is not None:
            _dict['classes'] = [x._to_dict() for x in self.classes]
        return _dict