如題，我希望在爬蟲一開始先讀取postgresql中的資料當作關鍵字，然後再存入mongoDB
如果需要實現這樣的狀況，請問該從何下手呢？
目前是使用postgresql讀取和存入。

spider.py

class SearchSpider(scrapy.Spider):
    def start_requests(self):
        session = pipelines.NewsSaveToPostgresPipeline().sessionmaker()
        total_sql = session.execute(text())

pipelines.py

class NewsSaveToPostgresPipeline(object):
    @classmethod
    def from_crawler(cls, crawler):
        cls.DB_CON_STR = crawler.settings.get('DB_CON_STR')
        cls.engine = create_engine(cls.DB_CON_STR, echo=True, poolclass=NullPool)  # 连接数据库
        cls.sessionmaker = sessionmaker(bind=cls.engine)
        Base.metadata.create_all(cls.engine)

        return cls()

    def open_spider(self, spider):
        self.session = self.sessionmaker()

    def close_spider(self, spider):
        self.session.close()

        def process_item(self, item, spider):
        if spider.name == 'proxy_example':
            self.session.add(Proxy(**item))
        else:
            self.session.add(WebData(**item))
        self.session.commit()