$lookup on ObjectId's in an array of objects (Mongoose)

I have this two schema:

module.exports = mongoose.model('Article', {
title : String,
text : String,
lang : { type: String, default: 'en'},
user : { type : mongoose.Schema.Types.ObjectId, ref: 'User' },
});



var userSchema = mongoose.Schema({
email        : String,
name         : String,
rating       : [{
                    _id: false,
                    articleID: {type: mongoose.Schema.Types.ObjectId, ref: 'Article'},
                    rate: Number
                }]
});
module.exports = mongoose.model('User', userSchema);

and i want to calculate the average rating of an user (the average on all rating on its articles).

I tried this:

User.aggregate([
            { $unwind: "$rating" },
            {
                "$lookup": {
                    "from": "Article",
                    "localField": "rating.articleID",
                    "foreignField": "_id",
                    "as": "article-origin"
                }
            }//,
            //{ $match: { "article-origin.user" : mongoose.Types.ObjectId(article.user) } }//,
            //{ $group : {_id : "$rating.articleID", avgRate : {  $avg : "$rating.rate" } } }
        ]).exec(function (err,result) {
            console.log(err);
            console.log(JSON.stringify(result));
        });

but without success, the lockup always return the field article-origin null.

Why this is not working ?

via Gabriele Picco