Use of $sum inside set - Mongodb - mongodb

Question

I have the following problem.
I want to create inside my mongodb database a new field for each documentation. This field should consist as a value the sum of two other fields of the documentation.
I tried this:
db.collection('restaurants').updateMany(
{}, { $set:
{ allScores: {$sum: "$grades.score"} }
}
)
but it doesn't work. I get the following error:
The dollar ($) prefixed field '$sum' in 'allScores.$sum' is not valid for storage.
Why can't I use $sum inside of $set?
And what can I do instead?
The database I used can be found here: https://www.w3resource.com/mongodb-exercises/
Thanks!
Julia
const MongoClient = require('mongodb').MongoClient;
ObjectID = require('mongodb').ObjectID;
// Connection URL
const url = 'mongodb://localhost:27017';
// Database Name
const dbName = 'myproject';
// Use connect method to connect to the server
MongoClient.connect(url, { useNewUrlParser: true }, function(error, client) {
if(error) {
return console.log('An error occured!')
}
console.log("Connected successfully to server");
const db = client.db(dbName);
var objectId = new ObjectID();
db.collection('restaurants').updateMany(
{}, { $set:
{ allScores: {$sum: "$grades.score"} }
}
)

Answer 1

You can use aggregate() with $out to do that:
db.collection('restaurants').aggregate([
{$addFields : {allScores : {$sum : "$grades.score"}}},
{$out : "restaurants"}
])
What this code does:
1- Find all documents inside restaurants;
2- Add the field allScores to each document;
3- Save all documents back to the collection restaurants;