Mongo db aggregation - $push and $slice top results - mongodb

Question

I have the following documents in my db:
{uid: 1, score: 10}
{uid: 2, score: 11}
{uid: 3, score: 1}
{uid: 4, score: 6}
{uid: 5, score: 2}
{uid: 6, score: 3}
{uid: 7, score: 8}
{uid: 8, score: 10}
I want to split them into buckets by score - i.e.:
score
uids
(bucket name in aggregation)
[0,4)
3,5,6
0
[4,7)
4
4
[7,inf
1,2,7,8
7
For this, I created the following aggregation which works just fine:
db.scores.aggregation(
[
{
$bucket:
{
groupBy: "$score",
boundaries: [0, 4, 7],
default: 7,
output:
{
"total": {$sum: 1},
"top_frustrated":
{
$push: {
"uid": "$uid", "score": "$score"
}
},
},
}
},
]
)
However, I would like to return only the top 3 of every bucket - i.e, buckets 0, 4 should be the same, but bucket 7 should have only uids 1,2,8 returned (as uid 7 has the lowest score) - but to include the total count of documents as well, i.e. output of bucket "7" should look like:
{ "total" : 4, "top_scores" :
[
{"uid" : 2, "score" : 11},
{"uid" : 1, "score" : 10},
{"uid" : 8, "score" : 10},
]
}
I tried using $addFields with $sortArray and $slice, but it either won't work or return errors.
I can of course use $project but I was wondering if there is a more efficient way.
I am using Amazon DocumentDB.

Answer 1

You can use the $topN accumulator, instead of $push, like this:
db.collection.aggregate([
{
"$bucket": {
"groupBy": "$score",
"boundaries": [
0,
4,
7
],
"default": 7,
"output": {
"total": {
"$sum": 1
},
"top_frustrated": {
"$topN": {
"n": 3,
"sortBy": {
"score": -1
},
"output": {
"uid": "$uid",
"score": "$score"
}
}
}
},
}
},
])
Playground link.
The only catch here is this operator is present in MongoDB 5.2 and above.
For older versions, this will work:
db.collection.aggregate([
{
"$sort": {
score: -1
}
},
{
$bucket: {
groupBy: "$score",
boundaries: [
0,
4,
7
],
default: 7,
output: {
"total": {
$sum: 1
},
"top_frustrated": {
$push: {
"uid": "$uid",
"score": "$score"
}
},
},
}
},
{
"$project": {
total: 1,
top_frustrated: {
"$slice": [
"$top_frustrated",
3
]
}
}
}
])
Playground link.