How to get a rating average in Mongoose / node (With rating in array of objects)

I need to calculate the rating average of an article, and the rating are in an array of objects

Here is my schema / model:

module.exports = mongoose.model('Article', {
    title : String,
    text : String,
    star : { type: Number, default: 3.5}
    ...
});

var userSchema = mongoose.Schema({
     email        : String,
     password     : String,
     name         : String,
     rating       : {type : Array, default: []}
});
module.exports = mongoose.model('User', userSchema);

where object in rating array are like { "rate" : "X", "articleID" : "Y" }

Now in this function i need to calculate the rating average

function recalculateArticleRate(articleIDD){
    User.find({'rating': {$elemMatch: {articleID : articleIDD}}}, function (err, result) {
        console.log(result);
    });
}

This is the result object:

[ { _id: 58f1448b92805b0cc2358536,
email: 'mail',
name: 'name',
__v: 0,
rating: 
 [ [Object],
   [Object],
   [Object],
   [Object]]
} ]

Doing this I get all users who evaluated the article, but how can I calculate the average directly with Mongoose?

via Gabriele Picco