How to get X students from each grade in MongoDB? - mongodb

Question

I have a mongoDB collection named students with the fields grade and name.
I would like to create a query that will get up to 2 students from each grade inside my gradesList.
So this is my input:
const gradesList = [10, 11, 12]
And this is my desired output:
[
{
name: 'Albert',
grade: 10
},
{
name: 'Tim',
grade: 10
},
{
name: 'Monika',
grade: 11
},
{
name: 'Mike',
grade: 11
},
{
name: 'Rich',
grade: 12
},
{
name: 'Beth',
grade: 12
},
]
I am assuming I need to use the aggregate framework but I have no idea how to accomplish what I am trying to do.
One solution is to iterate over the gradesList array and execute a query like this: db.students.find({ grade: 10 }).limit(2). However this is a really bad idea.
Is there an efficient way of accomplishing this in 1 query?

Answer 1

Here's one way to do it.
db.collection.aggregate([
{
"$match": {
"grade": {"$in": [10, 11, 12]}
}
},
{
"$group": {
"_id": "$grade",
"name": {
"$firstN": {
"n": 2,
"input": "$name"
}
}
}
},
{"$unwind": "$name"},
{
"$project": {
"_id": 0,
"name": 1,
"grade": "$_id"
}
},
{
"$sort": {
"grade": 1,
"name": 1
}
}
])
Sample output:
[
{
"grade": 10,
"name": "Jayne Cormier"
},
{
"grade": 10,
"name": "Rebekah Jacobi"
},
{
"grade": 11,
"name": "Mariano Reinger"
},
{
"grade": 11,
"name": "Shea Hartmann"
},
{
"grade": 12,
"name": "Colt Spinka"
},
{
"grade": 12,
"name": "Stephanie Schiller"
}
]
Try it on mongoplayground.net.