Mongoose: Calculate the number of grouped documents - node.js

Question

I have this mongoose query:
const filter_stage = {
$match: {
category: "LOGIN",
},
};
const active_users_group_stage = {
$group: {
_id: "$user",
number_of_logins: { $sum: 1 },
},
};
const pipeline = [filter_stage, active_users_group_stage];
const active_users_stats = await History.aggregate(pipeline);
response.active_users_stats = active_users_stats;
const number_of_active_users = active_users_stats.length;
This is the History model:
const HistorySchema = new Schema({
user: {
type: Schema.Types.ObjectId,
ref: "users",
},
category: {
type: String,
enum: ["LOGIN","LOGOUT"],
required: true,
},
date: {
type: Date,
default: Date.now,
},
});
And it returns something like this:
[
{
"_id": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
"number_of_logins": 45
},
{
"_id": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
"number_of_logins": 36
},
{
"_id": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
"number_of_logins": 26
},
{
"_id": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
"number_of_logins": 18
},
{
"_id": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
"number_of_logins": 18
},
]
Instead of calculating the number of active users like this:
const number_of_active_users = active_users_stats.length;
I would like to calculate it inside the aggregate pipeline.
Is this possible?

Answer 1

You need $group all with _id: null to sum all active_users and add all documents into the users array as the last stage.
{
$group: {
_id: null,
number_of_active_users: {
$sum: 1
},
users: {
$push: "$$ROOT"
}
}
}
As the active_users_stats result will return an array with only one document,
...
const all_group_stage = {
$group: {
_id: null,
number_of_active_users: {
$sum: 1
},
users: {
$push: "$$ROOT"
}
},
};
const pipeline = [filter_stage, active_users_group_stage, all_group_stage];
const active_users_stats = await History.aggregate(pipeline);
response.active_users_stats = active_users_stats[0].users;
const number_of_active_users = active_users_stats[0].number_of_active_users;