Grouping multiple documents with nested array of objects in MongoDB - mongodb

Question

I'm having documents that are having this structures
x = {
"scalar": 1,
"array": [
{"key": 1, "value": 2},
{"key": 2, "value": 3},
],
"array2": [
{"key": 1, "value": 2},
{"key": 2, "value": 3},
],
}
and
y = {
"scalar": 2,
"array": [
{"key": 1, "value": 3},
{"key": 3, "value": 0},
],
"array2": [
{"key": 1, "value": 3},
{"key": 3, "value": 0},
],
}
The end results I'm trying to find is this
{
"scalar": 3, # SUM of scalar
"array": [
{"key": 1, "value": 5}, # SUM by key = 1
{"key": 2, "value": 3},
{"key": 3, "value": 0},
],
"array2": [
{"key": 1, "value": 5}, # SUM by key = 1
{"key": 2, "value": 3},
{"key": 3, "value": 0},
],
}
I've tried to use double $unwind and then do push by. I'm thinking of using $reduce to get the final results

Answer 1

Query
one way to do it, is by facet, you want 3 groupings and facet can do that , like break into 3 seperate parts, to not mix the unwinds, i think this is the most simple way to do it
Test code here
db.collection.aggregate([
{
"$facet": {
"scalar": [
{
"$project": {
"scalar": 1
}
},
{
"$group": {
"_id": null,
"sum": {
"$sum": "$scalar"
}
}
},
{
"$unset": [
"_id"
]
}
],
"array": [
{
"$project": {
"array": 1
}
},
{
"$unwind": {
"path": "$array"
}
},
{
"$group": {
"_id": "$array.key",
"sum": {
"$sum": "$array.value"
}
}
},
{
"$project": {
"_id": 0,
"key": "$_id",
"value": "$sum"
}
}
],
"array2": [
{
"$project": {
"array2": 1
}
},
{
"$unwind": {
"path": "$array2"
}
},
{
"$group": {
"_id": "$array2.key",
"sum": {
"$sum": "$array2.value"
}
}
},
{
"$project": {
"_id": 0,
"key": "$_id",
"value": "$sum"
}
}
]
}
},
{
"$set": {
"scalar": {
"$arrayElemAt": [
"$scalar.sum",
0
]
}
}
}
])
Other alternative is to unwind both arrays, but then unwinds and groups will be mixed, making things complicated i think.
Also $reduce cant be used for grouping in MongoDB i think, because we can't construct dynamic paths.
If group-reduce and have this data (key=key value=value)
{"1" : 5 , "2" : 3}
And we see {"key" 1, "value" : 5} how we can check if the above data contains the 1 as key? We cant construct dynamic paths, like $$this.1 . Only way it to convert it to an array and back to object that will be so slow.