Mongoose calculating average from another collection - node.js

Question

I am new to MongoDB/Mongoose. I am trying to do a search based on a key string for a 'Resource' which will return a list of resources based on average of ratings for that resource. I am having a hard time calculating and returning the average. This is my schema.
Resource Schema:
const ResourceSchema = mongoose.Schema({
title: {
type: String,
required: true,
},
type: {
type: String,
required: true,
},
url: {
type: String,
required: true,
},
createdDate: {
type: Date,
default: Date.now,
},
});
module.exports = mongoose.model("Resource", ResourceSchema);
Rating Schema:
const RatingSchema = mongoose.Schema({
resourceId: {
type: mongoose.Schema.Types.ObjectId,
ref: "Resource",
},
createdDate: {
type: Date,
default: Date.now,
},
rating: {
type: Number,
required: true,
min: 1,
max: 5,
},
review: {
type: String,
required: true,
},
});
module.exports = mongoose.model("Rating", RatingSchema);
Each Resource will have multiple Ratings. I am trying to calculate the average of ratings in my list of fetched Resources and add it to the search results.
This is what I have for my search:
Resource.find({
$or: [
{ title: { $regex: req.params.searchStr.toLowerCase(), $options: "i" } },
{ url: { $regex: req.params.searchStr.toLowerCase(), $options: "i" } },
],
})

Answer 1

Here's one way you could do it.
db.resources.aggregate([
{ // filter resources
"$match": {
"title": {
"$regex": "passenger",
"$options": "i"
},
"url": {
"$regex": "https",
"$options": "i"
}
}
},
{ // get ratings for resource
"$lookup": {
"from": "ratings",
"localField": "_id",
"foreignField": "resourceId",
"pipeline": [
{
"$project": {
"_id": 0,
"rating": 1
}
}
],
"as": "ratings"
}
},
{ // calculate average
"$set": {
"avgRating": { "$avg": "$ratings.rating" }
}
},
{ // don't need ratings array anymore
"$unset": "ratings"
}
])
Try it on mongoplayground.net.