express.js + mongoose：回退到不正确的 ID？

Question

i use express.js with mongoose for mongoDB access. i try to implement a fallback, if someone enters an url which does not exist (based on the user_id). here's the code:

app.get('/track/:id', function(req, res) {
var user_id = req.params.id;
User.findById(user_id, function(err, user_) {
    if (!err) {
        res.render('track', { pins: user_.pins, layout: false });
    } else {
        res.send("FAIL! " + err);
    }
});
});

when accessing http://foo.bar/track/1234 and 1234 doesn't exist, it should print out FAIL,. if the id exist it should get it's content from the db (this actually works): this is what i get now when entering an incorrect id (the app crashes):

/home/node/node-service/releases/20110530081158/server.js:112
            res.render('track', { pins: user_.pins, layout: false });
                                             ^
TypeError: Cannot read property 'pins' of null
    at /home/node/node-service/releases/20110530081158/server.js:112:46
    at /home/node/.node_libraries/.npm/mongoose/1.3.6/package/lib/mongoose/query.js:778:22
    at [object Object].<anonymous> (/home/node/.node_libraries/.npm/mongoose/1.3.6/package/support/node-mongodb-native/lib/mongodb/collection.js:462:35)
    at [object Object].emit (events.js:67:17)
    at [object Object].<anonymous> (/home/node/.node_libraries/.npm/mongoose/1.3.6/package/support/node-mongodb-native/lib/mongodb/db.js:94:12)
    at [object Object].emit (events.js:64:17)
    at Socket.<anonymous> (/home/node/.node_libraries/.npm/mongoose/1.3.6/package/support/node-mongodb-native/lib/mongodb/connection.js:86:16)
    at Socket.emit (events.js:64:17)
    at Socket._onReadable (net.js:671:31)
    at IOWatcher.onReadable [as callback] (net.js:177:10)

Answer 1

Returning no data for an ID that doesn't exist is normal operation, not an error, and so err is not being set.

Try changing if (!err) to if (!err && user_) .

Answer 2

try switching the callback params >

User.findById(user_id, function(user_, err ) {
    if (!err) {
        res.render('track', { pins: user_.pins, layout: false });
    } else {
        res.send("FAIL! " + err);
    }
});