Populate playlist with songs Mongoose AngularJS

I am trying to use the "populate" function in mongoose to add song objects into playslists, but I can't get it working. When I create a new playlist, I only get:

{
    "_id": "54f468a0c3242661233bf933d",
    "id": 5,
    "name": "test5",
    "__v": 0,
    "playlistsongs": []
}

]

Question: How does the populate function know which songs to add to the playlist?

song.js

/*
*   Model for song
*/

// grab the mongoose module
var mongoose = require('mongoose');
var Schema = mongoose.Schema;

//define schema
var songSchema = new Schema({
    artist : { type: String, required: true },
    title : {type: String, required: true},
    genre : {type: String, required: true},
    url : { type: String, required: true }
});

// define our model
// module.exports allows us to pass this to other files when it is called
module.exports = mongoose.model('Song', songSchema);
exports.Song = mongoose.model('Song', songSchema);

playlist.js

/*
*   Model for playlist
*/

// grab the mongoose module
var mongoose = require('mongoose');
// create a new schema
var Schema = mongoose.Schema;

// define the schema
var playlistSchema = new Schema({
    name    : { type: String, required: true },
    id      : { type: Number, required: true},
    playlistsongs   : [{type: Schema.ObjectId, ref: 'Song'}]
});

// define our model
// module.exports allows us to pass this to other files when it is called
module.exports = mongoose.model('Playlist', playlistSchema);

routes.js

// route to get specific playlist
app.get('/api/playlists/:playlist_id', function(req, res, next) {
    Playlist.findById(req.params.playlist_id, function(err, playlist) {

        // if there is an error retrieving, send the error. 
        // nothing after res.send(err) will execute
        if (err)
            res.send(err);

        Playlist.
        find().
        populate("playlistsongs").
        exec(function(err, nws){
            if(err) {res.writeHead(500, err.message)}
            res.send(nws);
        });

        res.json(playlist); // return all playlists in JSON format
    });