fetch and modify , mongodb collection using mongoose related models

i have to populate a result json , from mongodb, using mongoose , in my case i am dealing with two collections named 'portfolio' and 'portfoliogroup'. I stored many rows in portfolio based on group selection. Now need to populate a data set with group name instead of group id.

//portfolio.js - model file

var mongoose = require('mongoose'),
    Schema = mongoose.Schema;

var portfoliolistSchema = new Schema({
    title: String,    
    desc: String,
    img: String,
    portgroup: [{ type: mongoose.Schema.Types.ObjectId, ref: 'portfoliogroup' }]
});

mongoose.model('portfoliolist', portfoliolistSchema);

//portfolio group model

 var mongoose = require('mongoose'),
    Schema = mongoose.Schema;

var portfolioGroupSchema = new Schema({
    name: String,    
});

mongoose.model('portfoliogroup', portfolioGroupSchema);

//express route

app.get('/pp', function (req, res) {
    var plist = [];
    portfoliolist.find().populate("portfoliogroup").exec(function (err, lists) {
res.json(lists)
    });

})

When i access the route im getting the following result json

[  
   {  
      "_id":"5511add1ba3221501f511788",
      "title":"to dodddd",
      "desc":"ddfsdfsdf",
      "img":"http://localhost:3000/media/336a1427221953256.jpg",
      "__v":0,
      "portgroup":[  
         "5511a88906dc9ef4074e0827"
      ]
   },
   {  
      "_id":"551646dfbe25faa01af8e9ac",
      "title":"New portfolio",
      "desc":"Small Desc",
      "img":"http://placehold.it/200x200",
      "__v":0,
      "portgroup":[  
          "5511a88906dc9ef4074e0828"
      ]
   }
]

but i need result josn should come with group name instead of group id

[  
   {  
      "_id":"5511add1ba3221501f511788",
      "title":"to dodddd",
      "desc":"ddfsdfsdf",
      "img":"http://localhost:3000/media/336a1427221953256.jpg",
      "__v":0,
      "portgroup":[  
         "Website"
      ]
   },
   {  
      "_id":"551646dfbe25faa01af8e9ac",
      "title":"New portfolio",
      "desc":"Small Desc",
      "img":"http://placehold.it/200x200",
      "__v":0,
      "portgroup":[  
            "ghraphic"    
      ]
   }
]

My environment is nodejs, expressjs,mongoose

any solution for this?