mongodb getting an array inside a nested array - mongodb

Question

I'm still new to mongodb, I have this basic enrollment system data:
{ "_id" : ObjectId("62277d92a561e550d5ec73ca"), "sid" : 1, "sname" : "sad", "semail" : "dsa", "scourse" : "it", "enrolled" : [ { "subjid" : 3 } ] }
{ "_id" : ObjectId("6227875bdbcc41a56a863697"), "sid" : 2, "sname" : "daws", "semail" : "dws", "scourse" : "cs", "enrolled" : [ { "subjid" : 1, "grades" : [ { "prelim" : "A", "midterm" : "B", "prefinal" : "B", "final" : "A" } ] }, { "subjid" : 2, "grades" : [ { "prelim" : "D", "midterm" : "A", "prefinal" : "B", "final" : "F" } ] } ] }
I want display the grades of sid 2 who has enrolled subjid 1.
I tried using this aggregation line:
db.students2.aggregate( [{"$match":{"sid":{"$eq":2},"enrolled.subjid":{"$eq":2}}}, {$group: {_id:'$enrolled.subjid[1]', prelim:{$first:'$enrolled.grades.prelim'},midterm:{$first:'$enrolled.grades.midterm'},prefinal:{$first:'$enrolled.grades.prefinal'},"final":{$first:'$enrolled.grades.final'} } } ])
but this was the result:
{ "_id" : [ ], "prelim" : [ [ "A" ], [ "D" ] ], "midterm" : [ [ "B" ], [ "A" ] ], "prefinal" : [ [ "B" ], [ "B" ] ], "final" : [ [ "A" ], [ "F" ] ] }
I only wanted to get the grades of subjid 1 but it also got the grades of subjid 2

Answer 1

Maybe you need something like this:
db.collection.aggregate([
{
"$match": {
"sid": 2,
"enrolled.subjid": 1
}
},
{
"$addFields": {
"enrolled": {
"$filter": {
"input": "$enrolled",
"as": "en",
"cond": {
$eq: [
"$$en.subjid",
1
]
}
}
}
}
},
{
$unwind: "$enrolled"
},
{
$unwind: "$enrolled.grades"
},
{$limit:1}
,
{
$project: {
_id: "$enrolled.subjid",
prelim: "$enrolled.grades.prelim",
midterm: "$enrolled.grades.midterm",
prefinal: "$enrolled.grades.prefinal",
"final": "$enrolled.grades.final"
}
}
])
Explained:
Match the necessary documents (sid=2,subjid=1)
Filter only the enrolled elements based on subjid ( subjid=1 )
unwind the two array
limit to the first result document available only in case there is more.
project the necesary fields
playground