My database is on mlab. I can connect to it using resteasy or by using localhost but when i try to deploy this code on heroku i get application error. I dont get it what I'm doing wrong

var express = require('express');
var app = express();
const bodyParser = require('body-parser');
const mongoose = require('mongoose');


app.use(express.static(__dirname+'/client'));
app.use(bodyParser.json());

Genre =require('./models/genres');
Book =require('./models/books');

app.set('port', (process.env.PORT || 61950));


app.use(express.static(__dirname + '/public'));

// views is directory for all template files
app.set('views', __dirname + '/views');
app.set('view engine', 'ejs');

app.get('/', function(request, response) {
  response.render('pages/index');
});

app.listen(app.get('port'), function() {
  console.log('Node app is running on port', app.get('port'));
});

// Connect to Mongoose mongoose.connect('mongodb://admin:admin@ds161950.mlab.com:61950/bookstore'); var db = mongoose.connection;

    app.get('/genres', (req, res) => {
    Genre.getGenres((err, genres) => {
        if(err){
            throw err;
        }
        res.json(genres);
    });
});