Return a json dictionary representing this model.

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