Implementing post('updateOne') hook middleware with Mongoose(MongoDB) - node.js

Question

I am trying to calculate the average value of the ratings for a Product per Document that is being rated. Instead of calculating the average rating of the Product every time when we need the value. I calculate it every time someone rates it. To accomplish this task I am implementing a post(‘updateOne’) hook middleware. Though not sure I can accomplish this by implementing this hook. Let me know if I am going into the wrong direction. Here is the error I am getting avgRating error from post updateOne hookTypeError: Cannot read property 'aggregate' of undefined.
file - Product.js
import mongoose from 'mongoose';
const { ObjectId } = mongoose.Schema;
const ParentSchema = new mongoose.Schema(
{
title: {
type: String,
trim: true,
required: true,
maxlength: 32,
text: true,
},
slug: {
type: String,
unique: true,
lowercase: true,
index: true,
},
price: {
type: Number,
required: true,
trim: true,
maxlength: 32,
},
quantity: Number,
ratings: [
{
star: Number,
postedBy: { type: ObjectId, ref: 'User' },
},
],
},
{
timestamps: true,
}
);
const avgRating = async function () {
try {
const stats = await this.aggregate([
{
$addFields: {
avgRating: { $avg: '$ratings.star' },
nRatings: { $size: '$ratings' },
},
},
]);
console.log('stats: ', stats);
} catch (err) {
console.log(`avgRating error from post updateOne hook${err}`);
}
};
// Call avgRating after updateOne
ParentSchema.post(
'updateOne',
{ document: true, query: false },
async function () {
await avgRating();
}
);
export default mongoose.models.Product ||
mongoose.model('Product', ParentSchema);
file - productServices.js
import Product from './Product.js';
const updateRating = async (existingRatingObject, star) => {
const query = {
ratings: { $elemMatch: existingRatingObject },
};
const update = { $set: { 'ratings.$.star': star } };
const option = { new: true };
try {
const doc = new Product();
const ratingUpdated = await doc.updateOne(query, update, option);
return ratingUpdated;
} catch (error) {
console.log('product model updateRating error: ', error);
}
};

Answer 1

Your avgRating function does not have access to the mongoose document under this. That is why you are getting the error, Cannot read property 'aggregate' of undefined, because this (the document) which you want to modify does not exists in the function.
You need to use an instance method available under mongoose. With instance method, you can call this to reference the document.
Check out:
https://www.freecodecamp.org/news/introduction-to-mongoose-for-mongodb-d2a7aa593c57/#instace-methods
https://mongoosejs.com/docs/guide.html#methods