MongoDB get $sum of fields created via $addFields - mongodb

Question

I'm trying to get sum of fields that were created with $addFields operator.
I'd like to get sum of fields for the first month among all documents.
Please see link to the MongoDB sandbox.
Data:
[
{
"key": 1,
"account": "a",
"cases_total_date": {
"20220101": 1,
"20220102": 2,
"20220103": 3,
"20220501": 4,
"20221201": 5,
"20221202": 6,
}
},
{
"key": 2,
"account": "b",
"cases_total_date": {
"20220101": 11,
"20220102": 12,
"20220103": 13,
"20220501": 14,
"20221201": 15,
"20221202": 16,
}
}
]
Query I've tried:
db.collection.aggregate([
{
"$match": {
"account": {
"$in": [
"a",
"b"
]
}
}
},
{
"$addFields": {
"cases_total_months|202201": {
"$sum": [
"$cases_total_date.20220101",
"$cases_total_date.20220102",
"$cases_total_date.20220103"
]
}
}
},
{
"$group": {
"_id": "",
"cases_total_months|202201_all": {
"$sum": "$cases_total_months|20220101"
}
}
}
])
The response I've got vs expected:
[
{
"_id": "",
"cases_total_months|202201_all": 0 # EXPECTED sum of fields from 2 docs 6+36=42
}
]
Would appreciate any feedback. Thank you!

Answer 1

Using dynamic values as field names is considered an anti-pattern and introduces unnecessary complexity to the queries. With a proper schema, you can do something simple as this:
db.collection.aggregate([
{
"$set": {
"cases_total_months|202201_all": {
"$filter": {
"input": "$cases_total_date",
"as": "ctd",
"cond": {
$and: [
{
$eq: [
2022,
{
$year: "$$ctd.date"
}
]
},
{
$eq: [
1,
{
$month: "$$ctd.date"
}
]
}
]
}
}
}
}
},
{
$group: {
_id: null,
"cases_total_months|202201_all": {
$sum: {
$sum: "$cases_total_months|202201_all.value"
}
}
}
}
])
Mongo Playground
For your current schema, you can still rely on $objectToArray and iterate through the resulting k-v tuples to get what you need.
db.collection.aggregate([
{
$set: {
cases_total_date: {
"$objectToArray": "$cases_total_date"
}
}
},
{
$set: {
"cases_total_months|202201_all": {
"$filter": {
"input": "$cases_total_date",
"as": "ctd",
"cond": {
$eq: [
0,
{
"$indexOfCP": [
"$$ctd.k",
"202201"
]
}
]
}
}
}
}
},
{
$set: {
"cases_total_months|202201_all": {
$sum: "$cases_total_months|202201_all.v"
}
}
},
{
$group: {
_id: null,
"cases_total_months|202201_all": {
$sum: "$cases_total_months|202201_all"
}
}
}
])
Mongo Playground