node.js - Mongoose/Express : average of subdocuments -

i have following models :

product :

var productschema = new schema({ name: string, comments: [{ type: schema.types.objectid, ref: 'comment'}], _user: { type: schema.types.objectid, ref: 'user'} }); 

comment :

var commentschema = new schema({ text: string, rating: number, _product: { type: schema.types.objectid, ref: 'product'} }); 

what retrieve products along user :

router.get('/', function(req, res, next) { product.find().populate('_user').exec(function (err, products) {     if (err) return next(err);     res.json(products); }); }); 

i'd add results "average" field contains average of comments each product results :

[{name: "product 1", _user: {name: "bob"}, average: 7.65},...] 

is possible unique query ? need compute , store average in product document each time new comment added ?

thanks !

maybe should try calculating "running average". need know how many ratings there are, , average. having same average value saved every document in mongodb should bad practice, hope you. create schema this:

var averageproductratingschema = new schema({   productid:       {type: schema.types.objectid, ref: 'product'},   averagerating:   {type: number},   numberofratings: {type: number} }); 

then implement addrating() function this:

function addrating(newrating, productid) {   /* find document holds average rating of wanted product */   averageproductrating.findoneasync({productid: productid})     .then(function (avgprodrating) {         /*           calculate new average using running average method.          http://www.bennadel.com/blog/1627-create-a-running-average-without-storing-individual-values.htm        */        var newaveragerating = (avgprodrating.averagerating * avgprodrating.numberofratings + newrating) / (avgprodrating.numberofratings + 1);        var newnumberofratings = avgprodrating.numberofratings + 1;         averageproductrating.update(          { productid: productid },          {            $set: {              averagerating:   newaveragerating,              numberofratings: newnumberofratings           }        });      });  } 

this link describes similiar issue: http://www.bennadel.com/blog/1627-create-a-running-average-without-storing-individual-values.htm