Aggregation for computing distribution across array - mongodb

Question

I'm trying to create an aggregation that will compute the distribution of values across an array of objects and return an array of computed values.
Here is a sample document
[
{
"duration": 1208,
"dataPoints": 2,
"binMin": 0,
"binMax": 5000
},
{
"duration": 25735,
"dataPoints": 3,
"binMin": 5000,
"binMax": 10000
},
{
"duration": 0,
"dataPoints": 0,
"binMin": 10000,
"binMax": 20000
},
{
"duration": 54088,
"dataPoints": 2,
"binMin": 20000,
"binMax": 28817
}
]
I need to add up the durations for each object, then compute the distribution across each object and return a new array like so:
[
{
"duration": 1208,
"dataPoints": 2,
"binMin": 0,
"binMax": 5000,
"ratio": 0.014907874763979
},
{
"duration": 25735,
"dataPoints": 3,
"binMin": 5000,
"binMax": 10000,
"ratio": 0.317594500870037
},
{
"duration": 0,
"dataPoints": 0,
"binMin": 10000,
"binMax": 20000,
"ratio": 0
},
{
"duration": 54088,
"dataPoints": 2,
"binMin": 20000,
"binMax": 28817,
"ratio": 0.667497624365983
}
]
I am able to calculate the total duration and divide to get the ratio value but it seems to be only doing it to the first element of the array.
This is my aggregation so far:
[{$project: {
_id: '$_id',
username: 1,
uuid: 1,
data: '$stats.dataHistogram'
}}, {$unwind: {
path: '$data'
}}, {$group: {
_id: '$_id',
data_bin: {
$first: '$data'
},
total_duration: {
$sum: '$data.duration'
}
}}, {$project: {
_id: '$_id',
total_duration: 1,
data_bin: 1,
ratio: {
$divide: [
'$data_bin.duration',
{
$add: [
'$total_duration',
1
]
}
]
}
}}]
(I'm adding a 1 to the $total_duration because it can be 0 some times and I get a "Cannot divide by zero" error)
I feel like I'm super close but not sure what the next steps should be. Thanks for the help!

Answer 1

You can use $reduce to compute the total duration first. Then apply element wise $divide by using $map
db.collection.aggregate([
{
"$addFields": {
"totalDuration": {
"$reduce": {
"input": "$stats.histogram",
"initialValue": 0,
"in": {
$add: [
"$$value",
"$$this.duration"
]
}
}
}
}
},
{
"$addFields": {
"totalDuration": {
"$cond": {
"if": {
$eq: [
"$totalDuration",
0
]
},
"then": 1,
"else": "$totalDuration"
}
}
}
},
{
"$addFields": {
"stats.histogram": {
"$map": {
"input": "$stats.histogram",
"as": "h",
"in": {
"duration": "$$h.duration",
"dataPoints": "$$h.dataPoints",
"binMin": "$$h.binMin",
"binMax": "$$h.binMax",
"ratio": {
"$divide": [
"$$h.duration",
"$totalDuration"
]
}
}
}
}
}
}
])
Here is the Mongo playground for your reference.