Node.js - TypeError: Cannot read property 'readPreference' of undefined

Question

I am getting a type error while trying to use aggregate method of mongodb in my Node program. I am using node 0.10.16 version and mongodb mongodb-win32-x86_64-2.4.2 version.

Here are the details of my 'relation' table in mongodb - `

db.relation.find().forEach(printjson)

{ "_id" : ObjectId("518808811189128048034c7b") }
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("5188137dee3a599c34000001")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2bd0afa8d2e82b000001")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2c81681aae0838000001")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2c90681aae0838000002")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "CSK",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2cefc7dc797c08000001")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2d48a2afbdc426000001")
}
{
        "username" : "51880fa37901642c36000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("518c2e2fa2afbdc426000002")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fda9ea6abb63037000002")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "who will IPL this year?",
        "_id" : ObjectId("521fdb3ba6abb63037000003")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fde8da6abb63037000004")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "who will IPL this year?",
        "_id" : ObjectId("521fdef0a6abb63037000005")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fdf1aa6abb63037000006")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fdffe257b59cc3f000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe09c827ff1e418000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe0cb5022f0c03c000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "Mumbai",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe1558b82555c25000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe17d8b82555c25000002")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe2e8c134b7bc40000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "CSK",
        "question" : "who will IPL this year?",
        "_id" : ObjectId("521fe354b59421cc07000001")
}
{
        "username" : "521fda91a6abb63037000001",
        "selectedoption" : "RCB",
        "question" : "Which team is going to win IPL6?",
        "_id" : ObjectId("521fe392b59421cc07000002")
}

This is all rough data in above table ( collection in mongodb ). I need a 'group by' in sql terms on the above table on the basis of 'selectedoption' column and just need a count on that. So the result will be a count on 'RCB', 'CSK', 'Mumbai' data. Here is my sample node snippet, removed un-required lines, I am using express server -

var express = require('express'),
  routes = require('./routes'),
  user = require('./routes/user'),
  http = require('http'),
  path = require('path');

var app = express();
var mongo = require('mongodb');
var db = new mongo.Db('voteapp', new mongo.Server("127.0.0.1", 27017, {
  auto_reconnect: true
}, {
  safe: false,
  strict: false
}))

app.get('/test', function(req, res) {
  res.send('works');
  db.collection('relation', function(err, collection){ 
    console.log('in');
    collection.aggregate({$group : {_id: "$selectedoption", selectedoption: {$sum: 1}}});
    console.log('out');
  });
});

The above line -

collection.aggregate({$group : {_id: "$selectedoption", selectedoption: {$sum: 1}}})

gives me the following error in node console

TypeError: Cannot read property 'readPreference' of undefined at Collection.aggregate (D:\mongo-js - 1\node_modules\mongodb\lib\mongodb\collection.js:1632:17)

at D:\mongo-js - 1\app.js:107:16
at Db.collection (D:\mongo-js - 1\node_modules\mongodb\lib\mongodb\db.js:461:44)
at D:\mongo-js - 1\app.js:105:6
at callbacks (D:\mongo-js - 1\node_modules\express\lib\router\index.js:161:37)
at param (D:\mongo-js - 1\node_modules\express\lib\router\index.js:135:11)
at pass (D:\mongo-js - 1\node_modules\express\lib\router\index.js:142:5)
at Router._dispatch (D:\mongo-js - 1\node_modules\express\lib\router\index.js:170:5)
at Object.router (D:\mongo-js - 1\node_modules\express\lib\router\index.js:33:10)
at next (D:\mongo-js - 1\node_modules\express\node_modules\connect\lib\proto.js:190:15)

- It runs fine in the mongodb console and gives me following output -

> db.relation.aggregate({$group : {_id: "$selectedoption", selectedoption: {$sum: 1}}});
{
        "result" : [
                {
                        "_id" : "CSK",
                        "selectedoption" : 9
                },
                {
                        "_id" : "Mumbai",
                        "selectedoption" : 10
                },
                {
                        "_id" : "RCB",
                        "selectedoption" : 8
                },
                {
                        "_id" : null,
                        "selectedoption" : 1
                }
        ],
        "ok" : 1
}

I would really appreciate any pointers or guidance to resolve this issue.

Answer 1

You need to implement a callback functionlike this:

collection.aggregate({$group : {_id: "$selectedoption", selectedoption: {$sum: 1}}}, 
function(err, result){ 
    //some stuff
});