TypeError: Cannot call method 'each' of undefined node.js

It's a continuation of question from here . My aim is to update all the documents with minimum score.

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

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

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

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);
    }
);
cursor.each(function(err,doc)
{
db.collection('students').update({'_id':doc._id},{$pull:{'scores':{'score':doc.minimum}}});
});
});

Using this i am getting error

node app.js 
undefined

/home/oroborus/node_modules/mongodb/lib/mongodb/mongo_client.js:475
          throw err
                ^

TypeError: Cannot call method 'each' of undefined
        at /home/oroborus/hw3-1/app.js:18:8
        at /home/oroborus/node_modules/mongodb/lib/mongodb/mongo_client.js:83:5
        at /home/oroborus/node_modules/mongodb/lib/mongodb/mongo_client.js:472:11
        at process._tickCallback (node.js:415:13)

According to previous programs written by me and the post above this looks correct then still why does this error persists ?

Thanks