intiliaze session in MongoClient.connect

I am trying to reuse the database object.The session intialization is working fine outside the Mongoconnect callback. But when I move it inside the call back , the session cookie is not being set in the browser . Any ideas??

var express=require('express');
var bodyParser=require('body-parser');
var cookieParser=require('cookie-parser');
var session=require('express-session');

var mongodb=require('mongodb'),
MongoClient = mongodb.MongoClient;
var MongoStore = require('connect-mongo')(session);

var util=require('util');
var assert=require('assert');
var db;



var app=express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended:true}));
app.use(cookieParser());


MongoClient.connect('mongodb://localhost:27017/mydb',function(err,database){
    if(!err)
    {


        app.use(session(
            { name:'sessionCookie',secret:'secret',saveUninitialized:true,resave:true ,
            store: new MongoStore({  db:database })
            }
            ));

        app.listen(3000);

    }

});

This seems to be a timing issue between the time the app object is intialized aand the session is intilaized. I chnaged the code a bit and the issue seems to be fixed. I am not entirely sure about the root cause. Feel free to chip in 

var express=require('express');
var bodyParser=require('body-parser');
var cookieParser=require('cookie-parser');
var session=require('express-session');

var mongodb=require('mongodb'),
MongoClient = mongodb.MongoClient;
var MongoStore = require('connect-mongo')(session);

var util=require('util');
var assert=require('assert');
var db;



var app;



MongoClient.connect('mongodb://localhost:27017/mydb',function(err,database){
    if(!err)
    {
app=express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended:true}));
app.use(cookieParser());
        app.use(session(
            { name:'sessionCookie',secret:'secret',saveUninitialized:true,resave:true ,
            store: new MongoStore({  db:database })
            }
            ));

        app.listen(3000);
    }

});