Compare and add two array inside nested arrays - node.js

Question

I have below array of array inside my document
{
items: [
["Tax", 10, 20, 30],
["FX Adjustments", 10, 20, 30],
["Tax", 10, 20, 30],
["FX Adjustments", 10, 20, 30]
]
}
I need to combine Tax with Tax and FX Adjustments with FX Adjustments.
Output I need
{
items: [
["Tax", 20, 40, 60],
["FX Adjustments", 20, 40, 60]
]
}
I tried but with no luck
db.collection.aggregate([
{
$project: {
items: {
$reduce: {
input: "$items",
initialValue: [],
in: {
$cond: [
]
}
}
}
}
}
])
Kindly help with this
Thank you!!!

Answer 1

You need to start with $unwind and then $group by first array element ($arrayElemAt). Then you can run $reduce like you tried but since your array is multidimensional, you need to wrap it with $map to represent the index. To get back the original data format you can group by null and use $concatArrays to put grouping _id as first array element:
db.collection.aggregate([
{
$unwind: "$items"
},
{
$group: {
_id: { $arrayElemAt: [ "$items", 0 ] },
values: { $push: { $slice: [ "$items", 1, { $size: "$items" } ] } }
}
},
{
$project: {
_id: 1,
values: {
$map: {
input: { $range: [ 0, { $size: { $arrayElemAt: [ "$values", 0 ] } } ] },
as: "index",
in: {
$reduce: {
input: "$values",
initialValue: 0,
in: { $add: [ "$$value", { $arrayElemAt: [ "$$this", "$$index" ] } ] }
}
}
}
}
}
},
{
$group: {
_id: null,
items: { $push: { $concatArrays: [ [ "$_id" ], "$values" ] } }
}
}
])
Mongo Playground

Answer 2

Try below aggregate pipeline
collectionName.aggregate([
{
$unwind: "$items"
},
{
$group: {
_id: {
$arrayElemAt: [
"$items",
0
]
},
sum1: {
$sum: {
$arrayElemAt: [
"$items",
1
]
}
},
sum2: {
$sum: {
$arrayElemAt: [
"$items",
2
]
}
},
sum3: {
$sum: {
$arrayElemAt: [
"$items",
3
]
}
}
}
},
{
$project: {
_id: null,
items: {
$map: {
input: {
$objectToArray: "$$ROOT"
},
as: "item",
in: "$$item.v"
}
}
}
},
{
$group: {
_id: null,
items: {
$push: "$items"
}
}
}
])
unwind the items array
group according to Tax(first position element of item using $arrayElemAt)
In project stage use map on objectToArray to get value of keys pushed into array
push the array using $group giving desired output
Output:
[
{
"_id": null,
"items": [
[
"Tax",
20,
40,
60
],
[
"FX Adjustments",
20,
40,
60
]
]
}
]