amazone/server/models/product.js

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const ProductSchema = new Schema({
  category: { type:Schema.Types.ObjectId, ref: 'Category' },
  owner: { type:Schema.Types.ObjectId, ref: 'Owner' },
  title: String,
  description: String,
  photo: String,
  price: Number,
  stockQuantity: Number,
  reviews: [{ type:Schema.Types.ObjectId, ref: 'Review' }]
}, {
  toObject: { virtuals: true },
  toJSON: { virtuals: true }
})

ProductSchema.virtual('averageRating').get(function () {
  if (this.reviews.length > 0) {
    let sum = this.reviews.reduce((total, review) => {
      console.log('review', review)
      return total + review.rating
    }, 0)
    
    return sum / this.reviews.length
  }

  return 0
})

module.exports = mongoose.model('Product', ProductSchema)