Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self,
                   'conversation_id') and self.conversation_id is not None:
            _dict['conversation_id'] = self.conversation_id
        if hasattr(self, 'system') and self.system is not None:
            _dict['system'] = self.system._to_dict()
        if hasattr(self, 'metadata') and self.metadata is not None:
            _dict['metadata'] = self.metadata._to_dict()
        if hasattr(self, '_additionalProperties'):
            for _key in self._additionalProperties:
                _value = getattr(self, _key, None)
                if _value is not None:
                    _dict[_key] = _value
        return _dict