Return a json dictionary representing this model.

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