var socketsRepo = require(your db handler repository class)();

base.io.on('connection', function (socket) {
    var region = socket.handshake.query.region || null;

    // the codes when connection just connected
    socketsRepo.insertSocket(socket.session.user._id, socket.id, region).then(function () {

        socket.on('disconnect', function () {
            socketsRepo.deleteSocket(socket.id).then(function () {
              // your code after it deleted the socket
            }, function (err) {
                console.error(err);
            });
        });
    }, function (err) {
        console.error(err);
        socket.disconnect();
    });
});