Pass nodejs post parameter to socket connection

I am trying to write simple nodejs socket program to display the list of user connected. I am having issues keeping username and socket.id mapped.

Here is server side code:

app.post('/login', function(req, res){
    var user = {
                  id: socket.id,//how to get this socket.id? 
                  username: req.body.username,
                  loggedInTime: new Date()
            };
    connectionsArray.push(user);
    res.render('main');

});

io.on('connection', function(socket){
  console.log('Connecting user ' + socket.id);

  if (connectionsArray.length > 0) {
        setInterval(function(){ 
                io.sockets.emit('connectionsArray', connectionsArray);
            },1000);
  }

  socket.on('disconnect', function() {

        connectionsArray = connectionsArray.map(function(conn){
                            return conn.id != socket.id;
                        });
        console.log('user disconnected with' + socket.id);
    });
});

via Rajeev Jayaswal