Mongodb: How to merge $facet output 2 by 2? - mongodb

Question

Playground:https://mongoplayground.net/p/YUV_fReyGsr
I have following query. I need to combine the result 2 by 2. Meaning I need to combine facet "1","2" as a result and facet "3","4" as another result. It's guaranteed that the number of facet will be even. Also, each pair of facet should get at most one record(it might not matter)
db.collection.aggregate([
{
"$facet": {
"1": [
{
$match: {
"ID": "2"
}
}
],
"2": [
{
$match: {
"array.ID": "2"
}
}
],
"3": [
{
$match: {
"array.ID": "4"
}
}
],
"4": [
{
$match: {
"ID": "4"
}
}
]
}
}
])
The expected result will be
[
{
"1": [
{
"ID": "1",
"array": [
{
"ID": "2",
"attribute1": "456"
},
{
"ID": "3",
"attribute1": "567"
}
],
"attr1": "123"
}
],
"2": [
{
"ID": "4",
"array": [
{
"ID": "5",
"attr1": "456"
}
],
"attr1": "123"
}
]
}
]

Answer 1

I was able to figure this out using $concatArrays operator, along with $project.
Live demo here
Database
[
{
"ID": "1",
"attr1": "123",
"array": [
{
"ID": "2",
"attribute1": "456"
},
{
"ID": "3",
"attribute1": "567"
}
]
},
{
"ID": "4",
"attr1": "123",
"array": [
{
"ID": "5",
"attr1": "456"
}
]
}
]
Query
db.collection.aggregate([
{
"$facet": {
"1": [
{
$match: {
"ID": "2"
}
}
],
"2": [
{
$match: {
"array.ID": "2"
}
}
],
"3": [
{
$match: {
"array.ID": "4"
}
}
],
"4": [
{
$match: {
"ID": "4"
}
}
]
}
},
{
"$project": {
_id: 0,
"1": {
"$concatArrays": [
"$1",
"$2"
]
},
"2": {
"$concatArrays": [
"$3",
"$4"
]
}
}
}
])
Result
[
{
"1": [
{
"ID": "1",
"_id": ObjectId("5a934e000102030405000000"),
"array": [
{
"ID": "2",
"attribute1": "456"
},
{
"ID": "3",
"attribute1": "567"
}
],
"attr1": "123"
}
],
"2": [
{
"ID": "4",
"_id": ObjectId("5a934e000102030405000001"),
"array": [
{
"ID": "5",
"attr1": "456"
}
],
"attr1": "123"
}
]
}
]