Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'response_type') and self.response_type is not None:
            _dict['response_type'] = self.response_type
        if hasattr(self, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self, 'time') and self.time is not None:
            _dict['time'] = self.time
        if hasattr(self, 'typing') and self.typing is not None:
            _dict['typing'] = self.typing
        if hasattr(self, 'source') and self.source is not None:
            _dict['source'] = self.source
        if hasattr(self, 'title') and self.title is not None:
            _dict['title'] = self.title
        if hasattr(self, 'description') and self.description is not None:
            _dict['description'] = self.description
        if hasattr(self, 'preference') and self.preference is not None:
            _dict['preference'] = self.preference
        if hasattr(self, 'options') and self.options is not None:
            _dict['options'] = [x._to_dict() for x in self.options]
        if hasattr(self, 'message_to_human_agent'
                  ) and self.message_to_human_agent is not None:
            _dict['message_to_human_agent'] = self.message_to_human_agent
        if hasattr(self, 'topic') and self.topic is not None:
            _dict['topic'] = self.topic
        if hasattr(self, 'dialog_node') and self.dialog_node is not None:
            _dict['dialog_node'] = self.dialog_node
        if hasattr(self, 'suggestions') and self.suggestions is not None:
            _dict['suggestions'] = [x._to_dict() for x in self.suggestions]
        return _dict