Update the existing collection from Aggregate Pipeline - database

Question

I am very new to MongoDB and have been trying to create a new field within my collection that is calculated using existing data.
Is there a way to add the field myRating to the movies collection?
Here is what I came up with.
db.movies.aggregate([
{$unwind: "$genres"},
{$project:{_id:0, title:1, genres:1,
durationScore: {$cond: {if: {$gte: ["$runtime", 90]}, then: 10, else: 5}},
yearScore: {$cond: {if: {$gte: ["$year", 1990]}, then: 10, else: 5}},
genreScore: {$switch:{branches:[
{
case: {$eq :["$genres", "Action"]}, "then": 30 ,
},
{
case: {$eq :["$genres", "Western"]}, "then": 20 ,
},
{
case: {$eq :["$genres", "Comedy"]}, "then": 5 ,
},
{
case: {$eq :["$genres", "Drama"]}, "then": 15 ,
},
],
default: 10
}},
directorScore: {$switch:{branches:[
{
case: {$eq :["$director", "Quentin Tarantino"]}, "then": 20 ,
},
{
case: {$eq :["$director", "Martin Scorsese"]}, "then": 20 ,
},
],
default: 10
}}
}},
{$addFields: { myRating: { $sum: [ "$yearScore", "$durationScore", "$genreScore", "$directorScore" ]}}},
])
Sample of Data.
{
"_id": {
"$oid": "60502686eb0d3e3e849677ef"
},
"title": "Once Upon a Time in the West",
"year": 1968,
"rated": "PG-13",
"runtime": 175,
"countries": [
"Italy",
"USA",
"Spain"
],
"genres": [
"Western"
],
"director": "Sergio Leone",
"writers": [
"Sergio Donati",
"Sergio Leone",
"Dario Argento",
"Bernardo Bertolucci",
"Sergio Leone"
],
"actors": [
"Claudia Cardinale",
"Henry Fonda",
"Jason Robards",
"Charles Bronson"
],
"plot": "Epic story of a mysterious stranger with a harmonica who joins forces with a notorious desperado to protect a beautiful widow from a ruthless assassin working for the railroad.",
"poster": "http://ia.media-imdb.com/images/M/MV5BMTEyODQzNDkzNjVeQTJeQWpwZ15BbWU4MDgyODk1NDEx._V1_SX300.jpg",
"imdb": {
"id": "tt0064116",
"rating": 8.6,
"votes": 201283
},
"tomato": {
"meter": 98,
"image": "certified",
"rating": 9,
"reviews": 54,
"fresh": 53,
"consensus": "A landmark Sergio Leone spaghetti western masterpiece featuring a classic Morricone score.",
"userMeter": 95,
"userRating": 4.3,
"userReviews": 64006
},
"metacritic": 80,
"awards": {
"wins": 4,
"nominations": 5,
"text": "4 wins & 5 nominations."
},
"type": "movie"
}

Answer 1

I would suggest you keep _id field in $project stage.
Without considering performance, simply iterating through the aggregate result and $set myRating field through updateOne using the _id field.
db.movies.aggregate([
...
{$project:{_id:1, title:1, genres:1,
...
]).forEach(result = > {
db.movies.updateOne(
{_id : result._id},
{$set : {myRating : {result.myRating}}
})
})

Answer 2

Starting in MongoDB 4.2, you can use the aggregation pipeline for update operations. Try this query:
db.movies.updateOne(
{ "_id": ObjectId("60502686eb0d3e3e849677ef") },
[
{
$set: {
myRating: {
$let: {
vars: {
durationScore: { $cond: { if: { $gte: ["$runtime", 90] }, then: 10, else: 5 } },
yearScore: { $cond: { if: { $gte: ["$year", 1990] }, then: 10, else: 5 } },
genreScore: {
$switch: {
branches: [
{ case: { $in: ["Action", "$genres"] }, "then": 30 },
{ case: { $in: ["Western", "$genres"] }, "then": 20 },
{ case: { $in: ["Comedy", "$genres"] }, "then": 5 },
{ case: { $in: ["Drama", "$genres"] }, "then": 15 }
],
default: 10
}
},
directorScore: {
$switch: {
branches: [
{ case: { $eq: ["$director", "Quentin Tarantino"] }, "then": 20 },
{ case: { $eq: ["$director", "Martin Scorsese"] }, "then": 20 }
],
default: 10
}
}
},
in: { $sum: ["$$yearScore", "$$durationScore", "$$genreScore", "$$directorScore"] }
}
}
}
}
]
);