Return a json dictionary representing this model.

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