MongoDb-Need Count of all documents before grouping in aggregate query - node.js

Question

I want count of all documents before doing grouping as show in my Query. The purpose of this if ai get count of all documents then I can calculate Percentage(%) of each individual students count. I tried with $count but after using that I'm not able to group my query. Is there any way 1st I can have counts of all documents after using $match function then store it to another variable without losing data.
This is the Query
const data = await AttendanceSchema.aggregate([
{ $match: { subjectID: mongoose.Types.ObjectId(`${req.params.Sid}`) } },
// Before Group I also want count of all documents
{ $group: { _id: "$studentID", count: { $sum: 1 } } },
{
$lookup: {
from: "students",
localField: "_id",
foreignField: "_id",
as: "student",
},
},
{
$project: {
"student.createdAt": 0,
"student.updatedAt": 0,
"student.__v": 0,
"student.password": 0,
},
},
]);
The data I'm getting from this query
{
"data": [
{
"_id": "635d40803352895afffdc294",
"count": 3,
"student": [
{
"_id": "635d40803352895afffdc294",
"name": "D R",
"email": "d#gmail.com",
"number": "9198998888",
"rollNumber": 202,
"departmentID": "635a8ca21444a47d65d32c1a",
"classID": "635a92141a081229013255b4",
"position": "Student"
}
]
},
{
"_id": "635eb8898dea5f437789b751",
"count": 4,
"student": [
{
"_id": "635eb8898dea5f437789b751",
"name": "V R",
"email": "v#gmail.com",
"number": "9198998899",
"rollNumber": 203,
"departmentID": "635a8ca21444a47d65d32c1a",
"classID": "635a92141a081229013255b4",
"position": "Student"
}
]
}
]
}
The desired output:
"data": [
{
"_id": "635d40803352895afffdc294",
"totalCount: 7, //This is what I want
"count": 3,
"student": [
{
"_id": "635d40803352895afffdc294",
"name": "D R",
"email": "d#gmail.com",
"number": "9198998888",
"rollNumber": 202,
"departmentID": "635a8ca21444a47d65d32c1a",
"classID": "635a92141a081229013255b4",
"position": "Student"
}
]
},
{
"_id": "635eb8898dea5f437789b751",
"totalCount: 7, //This is what I want
"count": 4,
"student": [
{
"_id": "635eb8898dea5f437789b751",
"name": "V R",
"email": "v#gmail.com",
"number": "9198998899",
"rollNumber": 203,
"departmentID": "635a8ca21444a47d65d32c1a",
"classID": "635a92141a081229013255b4",
"position": "Student"
}
]
}
]
}

Answer 1

You could add a few more stages to you aggregation pipeline to output what you want.
Here are the extra stages:
{
"$group": {
"_id": null,
"totalCount": {"$sum": "$count"},
"docs": {"$push": "$$ROOT"}
}
},
{
"$project": {
"docs": {
"$map": {
"input": "$docs",
"as": "doc",
"in": {
"$mergeObjects": [
"$$doc",
{"totalCount": "$totalCount"}
]
}
}
}
}
},
{"$unwind": "$docs"},
{"$replaceWith": "$docs"}
Try it on mongoplayground.net.