使用MongoDB的：类型错误：无法读取的不确定

Question

财产“集合”冉这个问题，而这样做https://www.freecodecamp.com/challenges/store-data-in-mongodb

错误信息被

swyx:~/workspace $ learnyoumongo run find.js 
/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/mongodb/lib/server.js:231 
     process.nextTick(function() { throw err; }) 
            ^

TypeError: Cannot read property 'collection' of undefined 
    at Exercise.<anonymous> (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/exercises/find/exercise.js:37:5) 
    at next (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:271:17) 
    at Exercise.end (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:277:5) 
    at Workshopper.end (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper/workshopper.js:191:12) 
    at Workshopper.done (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper/workshopper.js:323:19) 
    at Exercise.<anonymous> (/home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:160:14) 
    at /home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/workshopper-exercise/exercise.js:147:16 
    at /home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/exercises/find/exercise.js:20:21 
    at /home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/mongodb/lib/mongo_client.js:238:20 
    at /home/ubuntu/.nvm/versions/node/v4.6.1/lib/node_modules/learnyoumongo/node_modules/mongodb/lib/db.js:242:14 

还有其他类似的问题在这里，但他们都不是描述此确切的问题

供参考，这是上

var mongo = require('mongodb').MongoClient 
var age = process.argv[2] 

var url = 'mongodb://localhost:27017/learnyoumongo' 

mongo.connect(url, function(err, db) { 
    if (err) throw err 
    var parrots = db.collection('parrots') 
    parrots.find({ 
    age: { 
     $gt: +age 
    } 
    }).toArray(function(err, docs) { 
    if (err) throw err 
    console.log(docs) 
    db.close() 
    }) 
}) 

的（有效的）代码我是运行它

Answer 1

解决方案是我的mongodb服务器在一夜之间意外关机，甚至没有运行。要重新启动它，必须恢复（https://docs.mongodb.com/manual/tutorial/recover-data-following-unexpected-shutdown/）然后重新启动。