How do I group and count values by value range in MongoDB - mongodb

Question

I have the following documents in my MongoDB:
_id: ObjectId(...)
'timestamp': 2022-11-03T10:00:00.000+00:00
score: 1
_id: ObjectId(...)
'timestamp': 2022-11-03T09:00:00.000+00:00
score: 3
_id: ObjectId(...)
'timestamp': 2022-11-03T10:00:00.000+00:00
score: 6
_id: ObjectId(...)
'timestamp': 2022-11-03T10:00:00.000+00:00
score: 10
I want to make an aggregation that counts the score within the range of (gte)1-(lt)5 as poor, (gte)5-(lt)7 as ok, (gte)7-(lt)8.5 as good and (gte)8.5-(lte)10 as excellent.
So the result would look like this:
{
"data": [
{
"name": "excellent",
"count": 1
},
{
"name": "good",
"count": 0
},
{
"name": "ok",
"count": 1
},
{
"name": "poor",
"count": 2
}
]
}
How do I achieve that?

Answer 1

If you accept an answer only with documents that have a count, you can do:
db.collection.aggregate([
{$project: {
_id: {
$arrayElemAt: [
["poor", "ok", "good", "excellent"],
{$floor: {$divide: ["$score", 10]}}
]}
}},
{$group: {_id: "$_id", count: {$sum: 1}}}
])
Otherwise you need to create all categories:
db.collection.aggregate([
{$group: {
_id: 0,
excellent: {$sum: {$cond: [{$gte: ["$score", 30]}, 1, 0]}},
good: {$sum: {$cond: [{$and: [{$gte: ["$score", 20]}, {$lt: ["$score", 30]}]}, 1, 0]}},
ok: {$sum: {$cond: [{$and: [{$gte: ["$score", 10]}, {$lt: ["$score", 20]}]}, 1, 0]}},
poor: {$sum: {$cond: [{$lt: ["$score", 10]}, 1, 0]}}
}},
{$unset: "_id"},
{$project: {data: {$objectToArray: "$$ROOT"}}},
{$project: {
data: {$map: {
input: "$data",
in: {nmae: "$$this.k", count: "$$this.v"}
}}
}}
])
See how it works on the playground example