docs: function (options, callback) {
              MongoClient.connect(connString, function (err, db) {
                  var at = db.collection(collection);
                  at.findOne({
                      'appId': options.appId,
                      'appSecret': options.appSecret,
                      'expire': {'$gt': new Date()}
                  }, function (err, at) {
                      db.close();
                      if(at == null){
                          callback('error');
                      } else {
                          callback(err, at.token);
                      }
                  });
              });
          }