class ProjectnameDownloaderMiddleware(object):

    # Not all methods need to be defined. If a method is not defined,
    # scrapy acts as if the downloader middleware does not modify the
    # passed objects.
    def __init__(self):
        self.Client = pymongo.MongoClient()
        self.Mydb = self.Client['jinrong']
        self.Table = self.Mydb['jinrog_table']

    @classmethod
    def from_crawler(cls, crawler):
        # This method is used by Scrapy to create your spiders.
        s = cls()
        crawler.signals.connect(s.spider_opened, signal=signals.spider_opened)
        return s

    def process_request(self, request, spider):
        # Called for each request that goes through the downloader
        # middleware.
        sha1 = hashlib.sha1(request.url)
        sha1_url = sha1.hexdigest()
        My_Query = {'sha1_url':sha1_url}
        Mydoc = self.Table.find(My_Query)
        if Mydoc.count() == 0:
            self.Table.insert({'url':request.url,'sha1_url':sha1_url})
            return None
        else:
            return request.url