Why modified request in Express JS is not keeping its value between POST and GET requests?

for a test project I need to login with an user defined only by username, and load new page with that user. My code in the tamplate is:

        <form action="/login" method="post">
        username: <input type="text" name="username"><br>
        password: <input type="password" name="password">
        <button type="submit" style="float: right;">Login..</button>
        </form>

The Express code in app.js is:

app.set('port', process.env.PORT || 3000);
app.set('views', path.join(__dirname, 'views'));
app.use(function (req, res, next) {
    req.usersession = { "userId" : "myUser" };
    next();
})
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.json());
app.use(express.urlencoded());
app.use(express.methodOverride());
app.use(app.router);
app.use(express.static(path.join(__dirname, 'public')));
// development only
  if ('development' == app.get('env')) {
   app.use(express.errorHandler());
}
//routes
app.get('/project', routes.project); 
app.get('/dashboard', routes.dashboard);
app.get('/', routes.index);
app.post('/addProject', routes.addProject); 
app.post('/login', routes.login); 

and the /login function:

exports.login = function (req, res) {
var query = req.body.username;
Users.
findOne({ 'username' : query }).
exec(function (err, result) {
    if (err) res.send('Username not found'); 
   // exports.dashboard();     
    req.usersession = result._id;
});

the /dashboard

exports.dashboard = function (req, res) {
Project.
find( { "project._addedBy" : req.usersession.userId}).
select("project.projectName project._addedBy").
exec(function (err, result) {
    if (err) throw err;
    var data = { "rendData" : result };
    res.render('dashboard', data);
});
};

When the browser is redirected to /dashboard, it starts with the initial data for req.usersession - myUser. How could I pass the req.usersession between both pageloads?

Sorry if my terms are not right, but I am new to the web.

Thanks!