Return a json dictionary representing this model.

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