Aggregation: Grouping based on array element in MongoDB - mongodb

Question

I am new to MongoDB, trying to write an aggregation function such that my output for the input should be same as below
[
{
"_id": {
"month": 1,
"year": 2022
},
"childServices": [
{"service":"MCT Latency", "sli":99.9},
{"service":"MCT Packet Loss", "sli":99.9}
],
"service": "Network"
},
{
"_id": {
"month": 2,
"year": 2022
},
"childServices": [
{"service":"MCT Latency", "sli":98.9},
{"service":"MCT Packet Loss", "sli":99.9}
]
"service": "Network",
}
]
Tried with below, but it's not grouping each childService by date.
[{
$unwind: {
path: '$childServices'
}
}, {
$group: {
_id: {
month: {
$month: '$date'
},
year: {
$year: '$date'
}
},
service: {
$first: '$service'
},
childServices: {
$first: '$childServices.service'
},
sli: {
$avg: '$childServices.availability'
}
}
}, {
$sort: {
'_id.month': 1,
'_id.year': 1
}
}]
SAMPLE DATA
[{
"_id": {
"$oid": "62fc99c00f5b1cb61d5f1072"
},
"service": "Network",
"date": "01/02/2022 00:32:51",
"childServices": [
{
"service": "MCT Latency",
"availability": 99.9,
},
{
"service": "MCT Packet Loss",
"availability": 99.9,
}
},
{
"_id": {
"$oid": "62fc99df0f5b1cb61d5f1073"
},
"service": "Network",
"date": "02/02/2022 00:32:51",
"childServices": [
{
"service": "MCT Latency",
"availability": 98.3,
},
"service": "MCT Packet Loss",
"availability": 99.9,
}
}
]
Basically, I want to get into the childService > pick each service > group them by month+year and get their monthly avg.

Answer 1

Convert the date from a string to a date type, before grouping, like this:
db.collection.aggregate([
{
$unwind: {
path: "$childServices"
}
},
{
$addFields: {
date: {
"$toDate": "$date"
}
}
},
{
$group: { <---- Here we are grouping the data for each distinct combination of month, year and child service. This needs to be done because we are using $first accumulator
_id: {
month: {
$month: "$date"
},
year: {
$year: "$date"
},
service: "$childServices.service"
},
service: {
$first: "$service"
},
childServices: {
$first: "$childServices.service"
},
sli: {
$avg: "$childServices.availability"
}
}
},
{
"$group": { <-- In this group, we groupBy month and year, and we push the child services record into an array, using $push. This gives us, for every month and year, the average of all distinct childservices
"_id": {
month: "$_id.month",
year: "$_id.year"
},
"childServices": {
"$push": {
service: "$childServices",
sli: "$sli"
}
}
}
},
{
$sort: {
"_id.month": 1,
"_id.year": 1
}
}
])
Playground link.