TypeError: object is not a function in using MongoDB with Node.js

Question

I was going through question and it asked me calculate minimum value of score for each child in class. I wrote the following source code.
I am using Mongo 2.6.9 and Node v0.10.25 and ubuntu 14.04 LTS

var MongoClient=require('mongodb').MongoClient;
var server=require('mongodb').Server;

var mongoclient=new MongoClient(new server("localhost",27017));

mongoclient.connect("mongodb://localhost:27017/",function(err,db){
if(err) throw err;

var db=mongoclient.db('school');

cursor=db.collection('students').aggregate(
[
  {$match : {"scores.type" : "homework"}},
  {$unwind:"$scores"},
  {$group : {_id : '$name',
  'minimum' : { $min :"$scores.score"  }
}
}
]);
});

This aggregate query when run using node app.js gives this error

/home/oroborus/node_modules/mongodb/lib/mongodb/connection/base.js:246
        throw message;      
              ^
TypeError: object is not a function
    at /home/oroborus/node_modules/mongodb/lib/mongodb/collection/aggregation.js:317:7
    at /home/oroborus/node_modules/mongodb/lib/mongodb/db.js:1195:7
    at /home/oroborus/node_modules/mongodb/lib/mongodb/db.js:1903:9
    at Server.Base._callHandler (/home/oroborus/node_modules/mongodb/lib/mongodb/connection/base.js:453:41)
    at /home/oroborus/node_modules/mongodb/lib/mongodb/connection/server.js:488:18
    at MongoReply.parseBody (/home/oroborus/node_modules/mongodb/lib/mongodb/responses/mongo_reply.js:68:5)
    at null.<anonymous> (/home/oroborus/node_modules/mongodb/lib/mongodb/connection/server.js:446:20)
    at EventEmitter.emit (events.js:95:17)
    at null.<anonymous> (/home/oroborus/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:207:13)
    at EventEmitter.emit (events.js:98:17)

Which according to me is coming from the aggregate function. But when i run the same query in mongo terminal i get a proper output.

cursor=db.students.aggregate( [   {$match : {"scores.type" : "homework"}},   {$unwind:"$scores"},   {$group : {_id : '$name', 'minimum' : {   $min :"$scores.score"  } } } ]);
{ "_id" : "Myrtle Wolfinger", "minimum" : 35.99397009906073 }
{ "_id" : "Gennie Ratner", "minimum" : 34.50565589246531 }
{ "_id" : "Nobuko Linzey", "minimum" : 19.27081566886746 }
{ "_id" : "Flora Duell", "minimum" : 40.68238966626067 }
{ "_id" : "Shin Allbright", "minimum" : 52.68629677727286 }

Ques- What and where is the mistake and how to rectify it .
Thanks.

Ben · Accepted Answer · 2015-04-04 22:52:08Z

5

The last argument of collection.aggregate() needs to be a callback. The mongodb driver is expecting a function but your last argument is an object. That's why you're getting that error. Here is the revised code with the callback:

var MongoClient = require('mongodb').MongoClient;
var server = require('mongodb').Server;

var mongoclient = new MongoClient(new server("localhost", 27017));

mongoclient.connect("mongodb://localhost:27017/", function(err, db) {
  if (err) throw err;

  var db = mongoclient.db('school');

  cursor = db.collection('students').aggregate(
    [
        {$match: {"scores.type": "homework"}},
        {$unwind: "$scores"},
        {
            $group: {
                _id: '$name',
                'minimum': {$min: "$scores.score"}
            }
        }
    ], function(err, result) {   // callback
        console.dir(result);
        db.close();
    }
  );
});

edited Apr 4, 2015 at 22:52

answered Apr 4, 2015 at 22:34

Ben

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2 Comments

Saras Arya Over a year ago

That worked like a charm. I wandered like 10 post, if i wouldn't have posted it here, i would have never managed to get an answer. Thanks a ton. :)

Saras Arya Over a year ago

can you answer this

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