MongoDB | get max value from nested object - mongodb

Question

db.getCollection('post').find({'post_no':47}, {'comment': 1})
The resulting values are:
{
"_id" : ObjectId("5bc05c038e798ccb0309658b"),
"comment" : [
{
"comment_no" : 112
},
{
"comment_no" : 113,
"comment_group" : 1
},
{
"comment_no" : 116,
"comment_group" : 2
},
{
"comment_no" : 117,
"comment_group" : 3
},
{
"comment_group" : 4,
"comment_no" : 118
}
]
}
I want to get the maximum value 4 of the comment_group.
What can I do?
Thank you for your advice.

Answer 1

db.collection.aggregate(
// Pipeline
[
// Stage 1
{
$unwind: {
path: "$comment",
}
},
// Stage 2
{
$sort: {
"comment.comment_group": -1
}
},
// Stage 3
{
$limit: 1
},
]
);

Answer 2

You can try below aggregation
db.collection.aggregate([
{ "$project": {
"comment": {
"$map": {
"input": "$comment",
"in": {
"comment_no": "$$this.comment_no",
"comment_group": { "$ifNull": [ "$$this.comment_group", 0 ] }
}
}
}
}},
{ "$project": {
"comment": {
"$arrayElemAt": [
"$comment",
{
"$indexOfArray": [
"$comment.comment_group",
{ "$max": "$comment.comment_group" }
]
}
]
}
}}
])

Answer 3

You can alse use $reduce in the second project in Anthony Winzlet's answer.
{
"$project": {
"comment": {
'$reduce': {
'input': '$comment',
'initialValue': {'$arrayElemAt': ['$comment', 0]},
'in': {
'$cond': {
'if': {'$gt': ['$$this.comment_group', '$$value.comment_group']},
'then': '$$this',
'else': '$$value'
}
}
}
}
}
}