Heroku mongo as服务错误：无法确定服务器的状态

我尝试将mongodb用作一项服务，并从heroku服务开始，即dashboard.heroku.com/apps/scaleqamongo/resources。我尝试了多个插件，他们称之为mongolab，mongohq。我创建数据库，然后尝试将该服务地址添加到我的应用程序中

db: 'mongodb://username:password@candidate.19.mongolayer.com:10292,candidate.18.mongolayer.com:10292/app25406086',

但它给了我错误

Error: Cannot determine state of server
Error: No replica set primary available for query with ReadPreference PRIMARY
    at ReplSet.checkoutReader (/prod/app/dashboard/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/repl_set/repl_set.js:585:14)
    at Cursor.nextObject (/prod/app/dashboard/node_modules/mongoose/node_modules/mongodb/lib/mongodb/cursor.js:723:48)
    at Cursor.nextObject (/prod/app/dashboard/node_modules/mongoose/node_modules/mongodb/lib/mongodb/scope.js:22:20)
    at Collection.findOne (/prod/app/dashboard/node_modules/mongoose/node_modules/mongodb/lib/mongodb/collection/query.js:157:10)
    at /prod/app/dashboard/node_modules/connect-mongo/lib/connect-mongo.js:214:18
    at MongoStore._get_collection (/prod/app/dashboard/node_modules/connect-mongo/lib/connect-mongo.js:148:21)
    at MongoStore.get (/prod/app/dashboard/node_modules/connect-mongo/lib/connect-mongo.js:213:10)
    at Object.session [as handle] (/prod/app/dashboard/node_modules/express/node_modules/connect/lib/middleware/session.js:311:11)
    at next (/prod/app/dashboard/node_modules/express/node_modules/connect/lib/proto.js:193:15)
    at Object.methodOverride [as handle] (/prod/app/dashboard/node_modules/express/node_modules/connect/lib/middleware/methodOverride.js:48:5)

而我可以使用shell命令连接到数据库

mongo candidate.19.mongolayer.com:10292/app25406086 -u username -p password

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有什么问题？为什么我无法连接？

这是一个间歇性错误，在我启动服务器后出现了一段时间，但后来消失了。

此行为可能与有关。如何从服务器上删除会话？