MongoDB Aggregate keep results - mongodb

Question

Is there a way to keep results from a match under a new field, and under another new field some computed value?
I'm trying to extract a set of genres from a collection of movies, and also keep original results...
Document example:
{
"_id": "62e97ba6ec445b864fc3bc39",
"id": 19913,
"genres": [
"Comedy",
"Drama",
"Romance"
],
"imdb_id": "tt1022603",
"overview": "Tom, greeting-card writer and hopeless romantic...",
"title": "(500) Days of Summer",
"release_date": "2009-07-17",
}
Desired output:
{
result: [
... movies
]
categories: [
"Comedy",
"Drama",
"Romance"
]
}
What I have so far:
use('the_base');
function matchGenre(genre) {
return {
"$match": {
"genres": genre,
}
};
}
function limit(num) {
return {
"$limit": num
};
}
db.movie.aggregate([
matchGenre("Drama"),
limit(5),
{"$unwind": "$genres"},
{"$group": {
"_id": 0,
"gens": { "$addToSet": "$genres" }
}}
]);
My current result:
{
"_id": 0,
"gens": [
"Romance",
"Comedy",
"Thriller",
"Science Fiction",
"Fantasy",
"Drama",
"Crime",
"Action",
"Mystery",
"Adventure",
"Horror"
]
}

Answer 1

I would generally use facets.
Here's an example: https://mongoplayground.net/p/PbORyp4JaF5
db.collection.aggregate([
{
$facet: {
results: [
{
$match: {}
}
],
categories: [
{
$unwind: "$genres"
},
{
$sortByCount: "$genres"
}
],
release_date: [
{
$unwind: "$release_date"
},
{
$sortByCount: "$release_date"
}
]
}
}
])
I have taken the liberty to add an additional facet of release_date, and also ensure that there is a count present in each of the facets, as this is often helpful and required.