Finding top 3 students in each subject MongoDB - mongodb

Question

I have tried searching for ways to solve my problem, except that my database is set up differently,
My documents in my collection are something like this:
{name:"MAX",
date:"2020-01-01"
Math:98,
Science:60,
English:80},
{name:"JANE",
date:"2020-01-01"
Math:80,
Science:70,
English:79},
{name:"ALEX",
date:"2020-01-01"
Math:95,
Science:68,
English:70},
{name:"JOHN",
date:"2020-01-01"
Math:95,
Science:68,
English:70}
{name:"MAX",
date:"2020-06-01"
Math:97,
Science:78,
English:90},
{name:"JANE",
date:"2020-06-01"
Math:78,
Science:76,
English:66},
{name:"ALEX",
date:"2020-06-01"
Math:93,
Science:75,
English:82},
{name:"JOHN",
date:"2020-06-01"
Math:92,
Science:80,
English:50}
I want to find the top 3 students for each subject without regard for the dates. I only managed to find the top 3 students in 1 subject.
So i group the students by name first, and add a column for max scores of a subject. Math in this case. Sort it in descending order and limit results to 3.
db.student_scores.aggregate(
[
{$group:{
_id: "$name",
maxMath: { $max: "$Math" }}},
{$sort:{"maxMath":-1}},
{$limit : 3}
]
)
Is there any way to get the top 3 students for each subject?
So, it would be top 3 for math, top 3 for science, top 3 for english
{
Math:{MAX, JANE, JOHN},
Science:{JOHN, ALEX, JANE},
English:{JANE, MAX, JOHN}
}

Answer 1

I just applied your code 3 times, using $facet
If you prefer a more compact result add
{$project:{English:"$Eng._id", Science:"$sci._id", Math:"$math._id"}}
PLAYGROUND
PIPELINE
db.collection.aggregate([
{
"$facet": {
"math": [
{
$group: {
_id: "$name",
maxMath: {
$max: "$Math"
}
}
},
{
$sort: {
"maxMath": -1
}
},
{
$limit: 3
}
],
"sci": [
{
$group: {
_id: "$name",
maxSci: {
$max: "$Science"
}
}
},
{
$sort: {
"maxSci": -1
}
},
{
$limit: 3
}
],
"Eng": [
{
$group: {
_id: "$name",
maxEng: {
$max: "$English"
}
}
},
{
$sort: {
"maxEng": -1
}
},
{
$limit: 3
}
]
}
}
])

Answer 2

Your question is not clear, but i can predict 2 scenario,
Get repetitive students along with date:
$project to show required fields and convert subjects object to array using $objectToArray
$unwind subjects array
$sort by subjects name in descending order
$group by subject name and get array of students
$project to get latest 3 students from students array
db.collection.aggregate([
{
$project: {
name: "$name",
date: "$date",
subjects: {
$objectToArray: {
Math: "$Math",
Science: "$Science",
English: "$English"
}
}
}
},
{ $unwind: "$subjects" },
{ $sort: { "subjects.v": -1 } },
{
$group: {
_id: "$subjects.k",
students: {
$push: {
name: "$name",
date: "$date",
score: "$subjects.v"
}
}
}
},
{
$project: {
_id: 0,
subject: "$_id",
students: { $slice: ["$students", 3] }
}
}
])
Playground
Sum of all date's score (means unique students):
$group by name, and get sum of all subjects using $sum,
$project to convert subjects object to array using $objectToArray
$unwind subjects array
$sort by subjects name in descending order
$group by subject name and get array of students
$project to get latest 3 students from students array
db.collection.aggregate([
{
$group: {
_id: "$name",
Math: { $sum: "$Math" },
Science: { $sum: "$Science" },
English: { $sum: "$English" }
}
},
{
$project: {
subjects: {
$objectToArray: {
Math: "$Math",
Science: "$Science",
English: "$English"
}
}
}
},
{ $unwind: "$subjects" },
{ $sort: { "subjects.v": -1 } },
{
$group: {
_id: "$subjects.k",
students: {
$push: {
name: "$_id",
score: "$subjects.v"
}
}
}
},
{
$project: {
_id: 0,
subject: "$_id",
students: { $slice: ["$students", 3] }
}
}
])
Playground