mongoose aggregate model based on filter

I have a model of Review which has the field product and rating

I would like to find the sum of all ratings of a specified product and find the average by dividing by 5.

const mongoose = require('mongoose');

const ReviewSchema = mongoose.Schema({
  product: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Product',
    required: true,
  },
  rating: {
    type: Number,
    required: true,
  },
});

module.exports = mongoose.model('Review', ReviewSchema);

how would I aggregate through all ratings and filter by product to find the sum?

Answer

I’m assuming your using nodejs , here is how I would go about doing this

const ReviewsModel = require('../Models/Review')
router.get('/average',async (req,res)=>{ 
    try {
        const reviewsPromise = await ReviewsModel.find({}).populate('product')
        if(reviewsPromise[0] == undefined) throw new Error('no reviews')

        const reviewsOfSpecifiedProduct= reviewsPromise.filter(rev=> rev.product.id == specifiedProductId)
        const sumOfRatings =  reviewsOfSpecifiedProduct
                               .map(rev=>rev.rating)//map to list of only ratings 
                               .reduce((a, b)=> a + b , 0)
        const ratingsAverage = sumOfRatings / 5 
        

        res.json(ratingsAverage)
    } catch (error) {
        console.log(error)
        //handle the errors here 
    }
})