Add empty buckets in mongo aggregation - mongodb

Question

I have the following aggregation:
const buckets = await StatisticModel.aggregate([
{
$bucket: {
groupBy: '$ranking',
boundaries: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11],
},
},
])
Which returns the following object:
[
{ _id: 3, count: 6 },
{ _id: 4, count: 98 },
{ _id: 5, count: 81 },
{ _id: 6, count: 25 },
{ _id: 7, count: 4 }
]
How can I add the missing (empty) buckets?
This is a simple example but I have more complexe ones where I generate the boundaries and I want to return to the front-end all the buckets and not only the filled ones.

Answer 1

You can use below aggregation
db.collection.aggregate([
{ "$facet": {
"data": [
{ "$bucket": {
"groupBy": "$ranking",
"boundaries": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11]
}}
]
}},
{ "$addFields": {
"data": {
"$map": {
"input": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11],
"as": "i",
"in": {
"_id": "$$i",
"count": {
"$cond": [
{ "$eq": [{ "$indexOfArray": ["$data._id", "$$i"] }, -1] },
0,
{ "$arrayElemAt": ["$data.count", { "$indexOfArray": ["$data._id", "$$i"] }] }
]
}
}
}
}
}},
{ "$unwind": "$data" },
{ "$replaceRoot": { "newRoot": "$data" }}
])
But better to do with javascript
const array = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11]
const array2 = [
{ "_id": 3, "count": 6 },
{ "_id": 4, "count": 98 },
{ "_id": 5, "count": 81 },
{ "_id": 6, "count": 25 },
{ "_id": 7, "count": 4 }
]
array.map((ar) => {
const index = array2.map((e) => { return e._id }).indexOf(ar)
if (index === -1) {
array2.push({ _id: ar, count: 0 })
}
})
console.log(array2)

Answer 2

Starting in Mongo 5.1, it's a perfect use case for the new $densify aggregation operator:
// { ranking: 3, count: 6 }
// { ranking: 4, count: 98 }
// { ranking: 6, count: 25 }
// { ranking: 7, count: 4 }
db.collection.aggregate([
{ $densify: {
field: "ranking",
range: { step: 1, bounds: [0, 12] }
}},
{ $set: { count: { $cond: [ { $not: ["$count"] }, 0, "$count" ] } } }
])
// { ranking: 0, count: 0 } <=
// { ranking: 1, count: 0 } <=
// { ranking: 2, count: 0 } <=
// { ranking: 3, count: 6 }
// { ranking: 4, count: 98 }
// { ranking: 5, count: 0 } <=
// { ranking: 6, count: 25 }
// { ranking: 7, count: 4 }
// { ranking: 8, count: 0 } <=
// { ranking: 9, count: 0 } <=
// { ranking: 10, count: 0 } <=
// { ranking: 11, count: 0 } <=
This:
densifies documents ($densify) by creating new documents in a sequence of documents where certain values for a field (in our case field: "ranking") are missing:
the step for our densification is 1: range: { step: 1, ... } since our buckets are following each other with a size of 1.
and we densify within the range [0, 12]: bounds: [0, 12]
finally sets ($set) count to 0 only for new documents included during the densify stage ({ count: { $cond: [ { $not: ["$count"] }, 0, "$count" ] } })
Note that I'm assuming your buckets are of equal size (assuming the missing 10 in your list is an oversight).