MongoDB: aggregating by property names - mongodb

Question

I have a collection within which each document looks a bit like this:
{
"_id" : ObjectId("5b9fe6010c969210d442a377"),
"statuses" : {
"SEARCHING" : 3
},
"regions" : {
"eu-central-1" : 1,
"us-west-2": 2
}
}
I want to group and transform this into a result set like this:
eu-central-1|us-west-2
1|2
Is this possible in one step/query?

Answer 1

This is probably what you want:
db.collection.aggregate({
$project: {
"regions": { $objectToArray: "$regions" } // convert sub-document into an array of key-value-pairs in order to get hold of the field names
}
}, {
$unwind: "$regions" // flatten the "regions" array
}, {
$group: {
"_id": "$regions.k",
"count": { $sum: "$regions.v" } //
}
})
Alternatively, if you really want to get a pipe-delimited output here is what you'd do:
db.collection.aggregate({
$project: {
"result": {
$reduce: {
"input": { $objectToArray: "$regions" },
"initialValue": { k: "", v: "" }, // start with empty strings for both key and value
"in": {
k: { $concat: [ "$$value.k", "|", "$$this.k" ] }, // concatenate existing value with "|" followed by currently looked at value for both key and value
v: { $concat: [ "$$value.v", "|", { $substr: [ "$$this.v", 0, 1000 ] } ] } // substr is needed to convert an integer field into a string
//v: { $concat: [ "$$value.v", "|", { $toString: "$$this.v" } ] } // this works from MongoDB v4.0 onwards and looks a bit cleaner
}
}
}
}
}, {
$project: { // remove the leading "|"
"result.k": { $substr: [ "$result.k", 1, -1 ] },
"result.v": { $substr: [ "$result.v", 1, -1 ] }
}
})