rendering results of multiple DB/mongoose queries to a view in express.js

Question

given the async nature of mongoose (or sequelize, or redis) queries, what do you do when you have multiple queries you need to make before rendering the view?

For instance, you have a user_id in a session, and want to retrieve some info about that particular user via findOne. But you also want to display a list of recently logged in users.

exports.index = function (req, res) {
    var current_user = null

    Player.find({last_logged_in : today()}).exec(function(err, players) {
        if (err) return res.render('500');

        if (req.session.user_id) {
            Player.findOne({_id : req.session.user_id}).exec(function(err, player) {
                if (err) return;
                if (player) {
                    current_user = player
                }
            })
        }

        // here, current_user isn't populated until the callback fires 
        res.render('game/index', { title: 'Battle!',
                   players: players,
                   game_is_full: (players.length >= 6),
                   current_user: current_user
        });
    });
};

So res.render is in the first query callback, fine. But what about waiting on the response from findOne to see if we know this user? It is only called conditionally, so I can't put render inside the inner callback, unless I duplicate it for either condition. Not pretty.

I can think of some workarounds -

make it really async and use AJAX on the client side to get the current user's profile. But this seems like more work than it's worth.
use Q and promises to wait on the resolution of the findOne query before rendering. But in a way, this would be like forcing blocking to make the response wait on my operation. Doesn't seem right.
use a middleware function to get the current user info. This seems cleaner, makes the query reusable. However I'm not sure how to go about it or if it would still manifest the same problem.

Of course, in a more extreme case, if you have a dozen queries to make, things might get ugly. So, what is the usual pattern given this type of requirement?

JohnnyHK · Accepted Answer · 2012-11-06 04:53:37Z

4

Yep, this is a particularly annoying case in async code. What you can do is to put the code you'd have to duplicate into a local function to keep it DRY:

exports.index = function (req, res) {
    var current_user = null

    Player.find({last_logged_in : today()}).exec(function(err, players) {
        if (err) return res.render('500');

        function render() {
            res.render('game/index', { title: 'Battle!',
                       players: players,
                       game_is_full: (players.length >= 6),
                       current_user: current_user
            });
        }

        if (req.session.user_id) {
            Player.findOne({_id : req.session.user_id}).exec(function(err, player) {
                if (err) return;
                if (player) {
                    current_user = player
                }
                render();
            })
        } else {
            render();
        }
    });
};

However, looking at what you're doing here, you'll probably need to look up the current player information in multiple request handlers, so in that case you're better off using middleware.

Something like:

exports.loadUser = function (req, res, next) {
    if (req.session.user_id) {
        Player.findOne({_id : req.session.user_id}).exec(function(err, player) {
            if (err) return;
            if (player) {
                req.player = player
            }
            next();
        })
    } else {
        next();
    }
}

Then you'd configure your routes to call loadUser wherever you need req.player populated and the route handler can just pull the player details right from there.

edited Nov 6, 2012 at 4:53

answered Nov 6, 2012 at 3:03

JohnnyHK

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3 Comments

sbeam Over a year ago

thanks, seems simple enough. What about using async.parallel to pull all the results together? I tried it and it seems to work, but wonder if it's a good approach. Yours is a lot less obfuscated, for sure.

JohnnyHK Over a year ago

Yeah async.parallel doesn't work well here. I updated my answer to include a middleware solution that's probably a better fit for your use case.

sbeam Over a year ago

nice, that's reusable and cleaner thanks to chaining with next().

vipinlalrv · Accepted Answer · 2017-03-10 09:45:41Z

router.get("/",function(req,res){
    var locals = {};
    var userId = req.params.userId;
    async.parallel([
        //Load user Data
        function(callback) {
             mongoOp.User.find({},function(err,user){
                if (err) return callback(err);
                locals.user = user;
                callback();
            });
        },
        //Load posts Data
        function(callback) {
                mongoOp.Post.find({},function(err,posts){
               if (err) return callback(err);
                locals.posts = posts;
                callback();
            });
        }
    ], function(err) { //This function gets called after the two tasks have called their "task callbacks"
        if (err) return next(err); //If an error occurred, we let express handle it by calling the `next` function
        //Here `locals` will be an object with `user` and `posts` keys
        //Example: `locals = {user: ..., posts: [...]}`
         res.render('index.ejs', {userdata: locals.user,postdata: locals.posts})
    });

Obie · Accepted Answer · 2014-10-08 15:12:55Z

0

Nowadays you can use app.param in ExpressJS to easily establish middleware that loads needed data based on the name of parameters in the request URL.

http://expressjs.com/4x/api.html#app.param

answered Oct 8, 2014 at 15:12

Obie

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