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

Question

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) {
    console.log("Recalculate article rate");
    User.aggregate([
        { $match: { "rating.articleID" : articleIDD } },
        { $unwind: "$rating" },
        { $group : {_id : "$_id", avgRate : {  $avg : "$rating.rate" } } }
    ], function (err, result) {
        if (err) {
            console.log(err);
            return;
        }
        console.log(result);
    });
}

This is the result object:

[ { _id: 58f519acfcb29003b572048b, avgRate: 3 },
{ _id: 58f5093159c45002f10ea7da, avgRate: 3 } ]

Doing this I get the average of all users evalutation, but i want the average of the rating of all users when rating.articleID = articleIDD. How I can do that with Mongoose?

Possible duplicate of MongoDB - calculating average of nested array of objects' attributes — s7vr
– s7vr, Commented Apr 17, 2017 at 10:41
Replace your group stage with { $group : {_id : null, avgRate : { $avg : "$rating.rate" } } } — s7vr
– s7vr, Commented Apr 17, 2017 at 21:25

Gabriele Picco · Accepted Answer · 2017-04-17 22:32:08Z

0

function recalculateArticleRate(articleIDD) {
    User.aggregate([
        { $unwind: "$rating" },
        { $match: { "rating.articleID" : articleIDD } },
        { $group : {_id : null, avgRate : {  $avg : "$rating.rate" } } }
    ], function (err, result) {
        if (err) {
            console.log(err);
            return;
        }
    });
}

Put unwind before and _id = null (as recommended by Veeram) solved my problem !

answered Apr 17, 2017 at 22:32

Gabriele Picco

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