How to calculate percentage using MongoDB aggregation - mongodb

Question

I want to calculate percentage of with help of mongoDB aggregation,
My collection has following data.
subject_id
gender
other_data
1
Male
XYZ
1
Male
ABC
1
Male
LMN
2
Female
TBZ
3
Female
NDA
4
Unknown
UJY
I want output something like this:
[{
gender: 'Male',
total: 1,
percentage: 25.0
},{
gender: 'Female',
total: 2,
percentage: 50.0
},{
gender: 'Unknown',
total: 1,
percentage: 25.0
}]
I have tried various methods but none of them works, mainly unable to count total of Male, Female, Unknown summation(to calculate percentage). The trickiest part is there are only 4 members in above example but their subject_id may be repeated according to other_data
Thanks in Advance.

Answer 1

You can use this aggregation query:
First group by subject_id to get the different values (different persons).
Then use $facet to create "two ways". One to use $count and get the total number of docs, and other to get the documents grouped by gender.
Then with all desired values (grouped by gender and total docs) get the first element of the result from nDocs into $facet stage. $facet will generate an array and the value we want will be in the first position.
Later use $unwind to get every groupValue with the nDoc value
And last output the values you want using $project. To get the percentage you can $divide total/nDocs and $multiply by 100.
db.collection.aggregate([
{
"$group": {
"_id": "$subject_id",
"gender": {
"$first": "$gender"
}
}
},
{
"$facet": {
"nDocs": [
{
"$count": "nDocs"
},
],
"groupValues": [
{
"$group": {
"_id": "$gender",
"total": {
"$sum": 1
}
}
},
]
}
},
{
"$addFields": {
"nDocs": {
"$arrayElemAt": [
"$nDocs",
0
]
}
}
},
{
"$unwind": "$groupValues"
},
{
"$project": {
"_id": 0,
"gender": "$groupValues._id",
"total": "$groupValues.total",
"percentage": {
"$multiply": [
{
"$divide": [
"$groupValues.total",
"$nDocs.nDocs"
]
},
100
]
}
}
}
])
Example here