Can't use $match with mongoose and the aggregation framework

Question

This is my Schema:

var userschema = new mongoose.Schema({

  user: String,
  follow: [String],
  imagen: [{ 

              title: String,
              date: { type: Date, default: Date.now }

           }]
 });

And this is the code:

 usermodel.findOne({ user: req.session.user }, function (err, user){
  usermodel.aggregate({$unwind: '$imagen'}, 
                   {$match: { _id: { $in: user.follow } }}, 
                   {imagen: true}, 
                   {$sort: {'imagen.date': 1}}, 
                    function (err, images){

                     console.log(images);              

                      res.render('home.ejs', {

                       user: user,
                       following: images

                      });
   });
  });

The follow contains users's _id.

The code works, except when I include the $match. I use the $matchto filter the result, only getting the images of the user that I'm following, but the console.log show me that the result of the aggregate search is undefined, but when I don't write the $match query, I get the images, but I obtain all the images, not only the images of the user that I'm following.

Is there any solution for this...?

Thank's advance!

EDIT:

var express = require('express');
var MongoStore = require('connect-mongo')(express);
var fs = require('fs');
var mongoose = require('mongoose');

var app = express();
app.listen(9191);

var sessionStore = new MongoStore({db: 'session'});

app.configure(function(){

   app.use(express.bodyParser());
   app.set('views',__dirname + '/views');
   app.set('view engine', 'ejs');
   app.use(express.static(__dirname + '/public'));
   app.use(express.cookieParser());
   app.use(express.session({
     store: sessionStore,
     secret: 'secret'
   }));
   app.use(app.router);

});

var db = mongoose.createConnection('localhost', 'test');

var userschema = new mongoose.Schema({

  user: String,
  follow: [String],
  imagen: [{ 

              title: String,
              date: { type: Date, default: Date.now }

           }]
 });

var usermodel =  db.model('usermodel', userschema);
var ObjectId = require('mongoose').Types.ObjectId;

app.get('/', middleware.noses, function (req, res){

     res.render('home0.ejs');

});


app.get('/home', middleware.yeses, function (req, res){

  usermodel.findOne({ user: req.session.user }, function (err, user){

    if (user.follow.length != 0){

      usermodel.find({ _id: {$in: user.follow } }, { user: true }, function (err, users){

         var usernames = users.map(function(u){ return u.user });

          usermodel.aggregate({$match: { _id: { $in: user.follow.map(
                                           function(id){ return new ObjectId(id); })}}},
                                       {$unwind: '$imagen'}, 
                                       {imagen: true}, 
                                       {$sort: {'imagen.date': 1}}, 
                                        function (err, images){

                                           console.log(images);

                                          res.render('home.ejs', {

                                             user: user,
                                             following: images

                                        });
               });
            });

    }  else {

       res.render('home.ejs', {

              user: user,
              following: undefined

            });

     }

 });
});

EDIT:

[ { __v: 4,
   _id: 50fd9c7b8e6a9d087d000006,
   follow: ['50fd9cbd1322de627d000006', '50fd9d3ce20da1dd7d000006'],
   imagen: 
   [{ title: 'foo',
      _id: 50fd9ca2bc9f163e7d000006,
      date: Mon Jan 21 2013 20:53:06 GMT+0100 (CET) },
    { title: 'foot',
      _id: 50fda83a3214babc88000005,
      date: Mon Jan 21 2013 21:42:34 GMT+0100 (CET) }],
   user: 'Mrmangado' }

Answer 1

Mongoose doesn't do any schema-based casting of the arguments of aggregate, so you need to convert your user.follow array of string ids into an array of ObjectIds in the $match:

{$match: { _id: { 
    $in: user.follow.map(function(id){ return new mongoose.Types.ObjectId(id); })
}}},

^{NOTE: Do not use mongoose.Schema.Types.ObjectId for casting. It will not work.}

You should also move this $match to the beginning of the pipeline for efficiency.

UPDATE

Your other problem is you need to use a $project operator instead of just including a plain {imagen: true} object in the pipeline. Putting it all together and reordering for a more efficient pipeline, this worked for me with your data:

usermodel.aggregate(
    {$match: { _id: {
        $in: user.follow.map(function(id){ return new mongoose.Types.ObjectId(id); })
    }}},
    {$project: {imagen: true}},
    {$unwind: '$imagen'},
    {$sort: {'imagen.date': 1}},
    function (err, images){ ...

Answer 2

To anyone who stumbles upon this question hoping for a more 'generic' solution with the need to manually .map or do conversions/transformation manually, here a solution that may work for you:

Requirements: Your query object for the $match must fit into some schema you've created.

General Solution:

Model.aggregate([
   { $match: Model.where(yourQueryObject).cast(QueryModel); }
]);

The catch here is Mongoose Query API's Query#cast which can help you transforming your plain object into whatever the schema defines. This helps normalizing ObjectIDs, strings and numbers.

Possible Solution to this Question:

Considering only the $match phase:

{$match: usermodel.where({_id: { $in: user.follow }}).cast(usermodel) }}