langchain/memory/chat_message_histories/sql.py/SQLChatMessageHistory/add_message
class SQLChatMessageHistory:    def add_message(self, message: BaseMessage) -> None:
        """Append the message to the record in db"""
        with self.Session() as session:
            jsonstr = json.dumps(_message_to_dict(message))
            session.add(self.Message(session_id=self.session_id, message=jsonstr))
            session.commit()