MongoDB add grand total to sortByCount() in an aggregation pipeline - mongodb

Question

I have grouped all the users by country, but I would also like to have a row showing the grand total (users are tagged to a single country in our use case).
Data Model / Sample Input
The collection is filled with objects representing a country (name) and each contains a list of user objects in an array under users.
{ _id: ObjectId("..."),
name: 'SG',
type: 'COUNTRY',
increment: 200,
users:
[ ObjectId("..."),
ObjectId("..."),
...
Query
db.collection.aggregate([{$match:{type:"COUNTRY"}},{$unwind:"$users"},{$sortByCount:"$name"}])
Current Results
{ _id: 'SG', count: 76 }
{ _id: 'IN', count: 6 }
{ _id: 'US', count: 4 }
{ _id: 'FR', count: 3 }
{ _id: 'UK', count: 2 }
{ _id: 'RU', count: 1 }
{ _id: 'CO', count: 1 }
{ _id: 'DK', count: 1 }
{ _id: 'ID', count: 1 }
{ _id: 'PH', count: 1 }
Expected Results
{ _id: 'SG', count: 76 }
{ _id: 'IN', count: 6 }
{ _id: 'US', count: 4 }
{ _id: 'FR', count: 3 }
{ _id: 'UK', count: 2 }
{ _id: 'RU', count: 1 }
{ _id: 'CO', count: 1 }
{ _id: 'DK', count: 1 }
{ _id: 'ID', count: 1 }
{ _id: 'PH', count: 1 }
{ _id: null, count: 96 } <<< TOTAL COUNT ADDED
Any tips to achieve this without resorting to complex or dirty tricks?

Answer 1

You can also try using $facet to calculate counts by country name and total count, and then combine them together. Something like this:
db.collection.aggregate([
{
$match: {
type: "COUNTRY"
}
},
{
"$unwind": "$users"
},
{
"$facet": {
"groupCountByCountry": [
{
"$sortByCount": "$name"
}
],
"totalCount": [
{
"$group": {
"_id": null,
"count": {
"$sum": 1
}
}
}
]
}
},
{
"$project": {
array: {
"$concatArrays": [
"$groupCountByCountry",
"$totalCount"
]
}
}
},
{
"$unwind": "$array"
},
{
"$replaceRoot": {
"newRoot": "$$ROOT.array"
}
}
])
Here's the playground link.

Answer 2

I recommend just doing this in memory as the alternative is "hacky" but in order to achieve this in Mongo you just need to group all documents, add a new documents and unwind again, like so:
db.collection.aggregate([
{
$group: {
_id: null,
roots: {
$push: "$$ROOT"
},
sum: {
$sum: "$count"
}
}
},
{
$addFields: {
roots: {
"$concatArrays": [
"$roots",
[
{
_id: null,
count: "$sum"
}
]
]
}
}
},
{
$unwind: "$roots"
},
{
$replaceRoot: {
newRoot: "$roots"
}
}
])
Mongo Playground