Return a json dictionary representing this model.

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