How to retrieve the bucket boundaries when using mongo's $bucket aggregation? - mongodb

Question

I have a collection of 200,000+ records which include a float field amountAwarded (eg. 12345.67, 2342, 22 etc). I'm using MongoDB to aggregate these into buckets based on the following boundaries:
amountAwarded: [
{
$bucket: {
groupBy: '$amountAwarded',
boundaries: [0, 10000, 50000, 100000, 1000000, Infinity],
output: {
count: { $sum: 1 }
}
}
}
]
This works as expected and I get this output:
{
"amountAwarded": [
{
_id: 0,
count: 269
},
{
_id: 10000,
count: 67
},
// etc
]
}
What I really want is to reference the bucket boundaries in the output, eg:
{
"amountAwarded": [
{
_id: 0,
count: 269,
lowerBound: 0,
upperBound: 9999
}
]
}
This means I can construct a list on the frontend showing the buckets (eg. £0 - £9999).
The closest I've come is adding $min: "$amountAwarded" (and an equivalent $max) to the output, which gives me the upper/lower values for that field in the bucketed records. This isn't right though as the numbers are obviously from the data (eg. 8762) rather than the bucket bounds.
Is it possible to refer to the matched bucket boundaries inside the aggregation pipeline, or will I have to construct this manually after the facet is complete?

Answer 1

You define your boundaries yourself, so you can just addFields on the next stage using combination of indexOfArray and arrayElemAt.
Something like this:
db.collection.aggregate([
{
$bucket: {
groupBy: '$amountAwarded',
boundaries: [0, 10000, 50000, 100000, 1000000, Infinity],
output: {
count: { $sum: 1 }
}
}
},
{ $addFields: {
lowerBound: "$_id",
upperBound: { $arrayElemAt: [
[0, 10000, 50000, 100000, 1000000, Infinity],
{ $add: [
{ $indexOfArray: [
[0, 10000, 50000, 100000, 1000000, Infinity], "$_id"
] },
1
] }
] }
} }
])