How to extract grouped results from array in $group stage and return as separate fields? - mongodb

Question

I'm running an aggregation query, and the $group stage is as follows
$group:
{
_id:
{
year_month: { $dateToString: { "date": "$updated_at", "format": "%Y-%m" } }
,client_name: "$clients_docs.client_name"
,client_label: "$clients_docs.client_label"
,client_code: "$clients_docs.client_code"
,client_country: "$clients_docs.client_country"
,base_curr: "$clients_docs.client_base_currency"
,inv_curr: "$clients_docs.client_invoice_currency"
,dest_curr: "$store.destination_currency"
}
,total_vol: { $sum: "$USD_Value" }
,total_tran: { $sum: 1 }
}
It returns the correct results, and returns all the grouped results in the _id:{} array.
I now want to extract all those fields from the array and return them not within the array so I can more easily export the output to a spreadsheet.
I tried using this stage:
{
$project:
{
year_month: 1
,client_name: 1
,client_label: 1
,client_code: 1
,client_country: 1
,base_curr: 1
,inv_curr: 1
,dest_curr: 1
,total_vol: 1
,total_tran : 1
}
},
But that returned the same results as the $group stage:
{
"_id" : {
"year_month" : "2022-01",
"client_name" : "client A",
"client_label" : "client A",
"client_code" : NumberInt(0000),
"client_country" : "TH",
"base_curr" : "USD",
"inv_curr" : "USD",
"dest_curr" : "HKD"
},
"total_vol" : 100000,
"total_tran" : 100.0
}
I want the "year_month" through "dest_curr" fields at the same level as the "total_vol" and "total_tran", so that when the data is exported they all appear as separate columns (now it's all captured as one "_id" column, and a "total_vol" and "total_tran" column). What's the best way to do this?

Answer 1

From a terminology perspective, you currently have an embedded document (or nested fields) rather than an array.
The straightforward way to do this is to simply enumerate each field, eg:
"year_month": "$_id.year_month",
There are fancier ways to do this, but as you only have a handful of fields this should suffice. Working playground example here.
Edit
An alternative ("fancier") approach is to leverage the $replaceWith stage using the $mergeObjects operator inside of it. Then you can $unset the previous _id field afterwards. It would look like this:
db.collection.aggregate([
{
"$replaceWith": {
"$mergeObjects": [
"$$ROOT",
"$_id"
]
}
},
{
$unset: "_id"
}
])
Playground link here
I also fixed the earlier playground link that had a typo for the client_label field.