Process data before express renders page

Using node and express, I'm trying to process some information before the response renders the page, but I'm not having any luck doing so. Any suggestions?

app.get("/Category", function (request, response) {
    if (request.query.Id) {
        // get all the posts by categoryId

        ForumPost.find({categoryId: request.query.Id}, function (err, posts) {
            if (err) throw err;

            var usernames = {};

            for (i = 0; i < posts.length; i++) {
                User.findById(posts[i].userId, function (err, user) {
                    if (err) throw err;
                    var username = user.username;
                    usernames[i] = username;
                });
            }
            response.render("Category", {
                message: posts,
                userList: usernames
            });

        });
    }
});

via JesseS102