Can I use "$first" operator on two fields in a "$group" operation in mongo db? - mongodb

Question

Consider the dataset
{ "_id" : { "$oid" : "aaa" }, "student_id" : 0, "type" : "exam", "score" : 54.6535436362647 }
{ "_id" : { "$oid" : "bbb" }, "student_id" : 0, "type" : "quiz", "score" : 31.95004496742112 }
{ "_id" : { "$oid" : "ccc" }, "student_id" : 0, "type" : "homework", "score" : 14.8504576811645 }
{ "_id" : { "$oid" : "ddd" }, "student_id" : 0, "type" : "homework", "score" : 63.98402553675503 }
{ "_id" : { "$oid" : "eee" }, "student_id" : 1, "type" : "exam", "score" : 74.20010837299897 }
{ "_id" : { "$oid" : "fff" }, "student_id" : 1, "type" : "quiz", "score" : 96.76851542258362 }
{ "_id" : { "$oid" : "ggg" }, "student_id" : 1, "type" : "homework", "score" : 21.33260810416115 }
{ "_id" : { "$oid" : "hhh" }, "student_id" : 1, "type" : "homework", "score" : 44.31667452616328 }
Say, for each student, I need to find minimum score and the corresponding document_id(_id).
Here is my pipeline
pipeline = [
{"$sort":{"student_id":1,"score":1 } },
{"$group": {"_id":"$student_id","mscore":{"$first":"$score"},"docid":{"$first":"$_id"} } },
{"$sort":{"_id":1}},
{"$project":{"docid":1,"_id":0}}
]
While this is working fine, I am not sure whether it is because I have the right query or whether it is because of way data is stored.
Here is my stragery
Sort by student_id, score
Group by student_id and do first on score, it will give student_id, min_score
Now, I need the doc_id(_id) also for this min_score, so I am using first on that field also. Is that correct?
Let's say after the sort, I need the entire first document, so should I apply first on each and every field or is there other way to do this?

Answer 1

To get the entire first document after sorting, apply the $first operator on the system variable $$ROOT which references the root document, i.e. the top-level document, currently being processed in the $group operator pipeline stage. Your pipeline would look like this:
var pipeline = [
{
"$sort": { "score": 1 }
},
{
"$group": {
"_id": "$student_id",
"data": { "$first": "$$ROOT" }
}
},
{
"$project": {
"_id": "$data._id",
"student_id": "$data.student_id",
"type": "$data.type",
"lowest_score": "$data.score"
}
}
]