How to aggregate percentages within arrays? - mongodb

Question

I'm trying to work out exactly how to achieve an aggregation, I could manually unwind and group back together at the end, but I'm sure I should be able to achieve this in a more concise way so I wanted to throw it out as I'm getting stuck.
My document structure (skipping out the un-interesting bits) looks like:
{
_id: ObjectId,
panels: [
{
visConfig: {
dataConfig: {
columns: [
{ element: "DX" },
{ element: "SE" },
]
}
}
},
{
visConfig: {
dataConfig: {
columns: [
{ element: "AB" },
{ element: "XY" },
]
}
}
}
]
}
What I want to do is calculate a percentage of the element overlaps with a given set to be provided. So for example for the document shown it would produce 25% for the set ["DX"] or 50% for the set ["DX", "AB"].
So I've tried a few things, I think I've settled on the nearest so far as:
$project: {
_id: 1,
total: { $sum: { $size: "$panels.visConfig.dataConfig.columns" } }
}
But I'm getting an error here which I don't understand:
The argument to $size must be an array, but was of type: missing
Then I'm also having issues with my conditional aggregation which seems to be returning 0 for all of the element values.
{
_id: 1,
"panels.visConfig.dataConfig.columns.element": {
$sum: {
$cond: [{
$setIsSubset: [
["DX"], ["$panels.visConfig.dataConfig.columns.element"]
]
}, 1, 0 ],
}
},
}

Answer 1

You can try below aggregation in 3.4 version.
db.colname.aggregate([
{"$project":{
"_id":1,
"total":{
"$reduce":{
"input":"$panels.visConfig.dataConfig.columns.element",
"initialValue":0,
"in":{"$add":["$$value",{"$size":"$$this"}]}
}},
"match":{
"$sum":{
"$map":{
"input":"$panels.visConfig.dataConfig.columns.element",
"in":{
"$size":{
"$setIntersection":[["DX","AB"],"$$this"]
}
}
}
}
}
}},
{"$project":{
"_id":1,
"percent":{"$multiply":[{"$divide":["$match","$total"]}, 100]}
}}])
Update - You can perform both match and total calculations in $reduce pipeline.
db.colname.aggregate([
{"$project":{
"_id":1,
"stats":{
"$reduce":{
"input":"$panels.visConfig.dataConfig.columns.element",
"initialValue":{"total":0,"match":0},
"in":{
"total":{"$add":["$$value.total",{"$size":"$$this"}]},
"match":{"$add":["$$value.match",{"$sum":{"$map":{"input":"$$this","in":{"$cond":[{"$in":["$$this", ["DX","AB"]] }, 1, 0]}}}}]}
}
}}
}},
{"$project":{
"_id":1,
"percent":{"$multiply":[{"$divide":["$stats.match","$stats.total"]}, 100]}
}}])

Answer 2

You can use $map + $reduce to get an array of all element values and then using $divide you can divide $filter-ed $size by total $size:
db.col.aggregate([
{
$project: {
elements: {
$reduce: {
input: {
$map: {
input: "$panels",
as: "panel",
in: "$$panel.visConfig.dataConfig.columns.element"
}
},
initialValue: [],
in: { $concatArrays: [ "$$this", "$$value" ] }
}
}
}
},
{
$project: {
percentage: {
$divide: [
{
$size: {
$filter: {
input: "$elements",
as: "element",
cond: {
$in: [
"$$element",
[ "AB", "XY" ] // your input here
]
}
}
}
},
{ $size: "$elements" }
]
}
}
}
])

Answer 3

Well, there are couple of ways to do this, but I these two pipelines show how I would do it.
var values = ["DX", "KL"]
First approach
[
{
"$project": {
"percent": {
"$let": {
"vars": {
"allsets": {
"$reduce": {
"input": "$panels.visConfig.dataConfig.columns",
"initialValue": [],
"in": {
"$concatArrays": [ "$$this.element", "$$value" ]
}
}
}
},
"in": {
"$multiply": [
{
"$divide": [
{
"$size": {
"$setIntersection": [ "$$allsets", values ]
}
},
{ "$size": "$$allsets" }
]
},
100
]
}
}
}
}
}
]
Second approach same idea here but, using one pipeline stage.
[
{
"$project": {
"percent": {
"$multiply": [
{
"$divide": [
{
"$sum": {
"$map": {
"input": "$panels.visConfig.dataConfig.columns.element",
"in": {
"$size": {
"$setIntersection": [ values, "$$this" ]
}
}
}
}
},
{
"$reduce": {
"input": "$panels.visConfig.dataConfig.columns.element",
"initialValue": 0,
"in": {
"$add": [ "$$value", { "$size": "$$this" } ]
}
}
}
]
},
100
]
}
}
}
]