Nested grouping in MongoDB aggregation - mongodb

Question

Context:
I have a MongoDB full of Documents like this, which I want to dumb into one grouped json:
[
{
"_id": "615dc97907f597330c510279",
"code": "SDFSDFSDF",
"location": "ABC1",
"week_number": 40,
"year": 2021,
"region": "NA"
},
....
{
"_id": "615dc97907f597330c51027a",
"code": "SDFSGSGR",
"location": "ABC1",
"week_number": 40,
"year": 2021,
"region": "EU"
},
....
{
"_id": "615dc97607f597330c50ff50",
"code": "GGSFHSFS",
"location": "DEF2",
"week_number": 40,
"year": 2021,
"region": "EU",
"audit_result": {
"issues_found": true,
"comment": "comment."
}
}
]
I am trying to write an aggregation which should return and object like this:
{
[
"EU": {
2021: {
40: {
"ABC1": {
(All documents for location ABC1 and week 40, year 2021 and region EU)
}
},
39: {
....
}
},
2020: {
....
}
},
"NA": {
....
}
]
}
Problem:
I am not 100% sure how.
I started grouping them by region but I am not sure how to proceed after the first group.
I tried grouping them by location first and group my way up to region but that also does not seem to work as I expected it.
The docs don't talk about a case like this and examples I find only group by one or two things, not four.
any insights highly appreciated :)

Answer 1

Using dynamic values as field name is generally considered as anti-pattern and you should avoid that. You are likely to introduce unnecessary difficulty to composing and maintaining your queries.
Nevertheless, you can do the followings in an aggregation pipeline:
$group at the finest level: region, year, week_number, location; $addToSet to group all the $ROOT document into an array named v
$group at 1 coarser level: region, year, week_number; create k-v tuples that k is the location and v is the v from step 1. Use $addToSet to group the k-v tuples into an array named v
use $arrayToObject to convert your k-v tuples into fields with dynamic values e.g.
"ABC" : [
{
"_id": "615dc97907f597330c510279",
...
},
...
]
Basically repeating step 2 & 3 at 1 coarser level: region, year; create k-v tuples that k is the location and v is the v from step 3. Use $addToSet to group the k-v tuples into an array named v
Repeat step 4 at 1 coarser level: region
$group unconditionally (i.e. $group by _id: null); repeating previous step to put the results into a single array named v; use $arrayToObject to convert it again
$replaceRoot to obtain your expected result
Here is one small note: when $arrayToObject for numeric k value like year and week_number, the k value needs to be converted into String beforehand. You can use $toString to achieve this.
Here is the Mongo playground for your reference.