Simple MongoDB Aggregation - mongodb

Question

I'm a bit confused on how to group using aggregation but still be able to extract specific values from arrays:
db.collection.aggregate([
{ "$unwind": f"${stat_type}" },
{
"$group": {
"_id": "$userId",
"value" : { "$max" : f"${stat_type}.stat_value" },
"character" : f"${stat_type}.character_name", <-- how do I extract this value that matches where the $max from above is grabbed.
}
},
{ "$sort": { "value": -1 }},
{ '$limit' : 30 }
])
Sample Entries:
{
'name' : "Tony",
'userId' : 12345,
'damage_dealt' : [
"character_name" : "James",
"stat_value" : 100243
]
}
{
'name' : "Jimmy",
'userId' : 12346,
'damage_dealt' : [
"character_name" : "James",
"stat_value" : 1020243
]
}
{
'name' : "Tony",
'userId' : 12345,
'damage_dealt' : [
"character_name" : "Lebron",
"stat_value" : 99900243
]
}
A sample output for what I'm looking for is below:
[
{
'_id':12345,
'user' : 'Tony'
'character_name' : 'Lebron',
'stat_value' : 99900243
},
{
'_id':12346,
'user' : 'Jimmy'
'character_name' : 'James',
'stat_value' : 1020243
}
]

Answer 1

You can use the $top accumulator to achieve the desired result. Like this:
db.collection.aggregate([
{
"$unwind": "$damage_dealt"
},
{
"$group": {
"_id": "$userId",
"value": {
$top: {
output: {
character_name: "$damage_dealt.character_name",
stat_value: "$damage_dealt.stat_value"
},
sortBy: {
"damage_dealt.stat_value": -1
}
}
},
}
},
{
"$project": {
character_name: "$value.character_name",
stat_value: "$value.stat_value"
}
},
{
"$sort": {
"stat_value": -1
}
},
{
"$limit": 30
}
])
Playground link.
Or collects all the group elements in an array, and the max stat_value, then pick the object from the array containing the max stat_value.
db.collection.aggregate([
{
"$unwind": "$damage_dealt"
},
{
"$group": {
"_id": "$userId",
"max_stat": {
"$max": "$damage_dealt.stat_value"
},
"damages": {
"$push": {
name: "$name",
damage_value: "$damage_dealt"
}
}
}
},
{
"$project": {
"damages": {
"$arrayElemAt": [
{
"$filter": {
"input": "$damages",
"as": "damage",
"cond": {
"$eq": [
"$$damage.damage_value.stat_value",
"$max_stat"
]
}
}
},
0
]
}
}
},
{
"$project": {
"character_name": "$damages.damage_value.character_name",
"stat_value": "$damages.damage_value.stat_value",
"name": "$damages.name"
}
},
{
"$sort": {
"stat_value": -1
}
},
{
"$limit": 30
}
])
Playground link.

Answer 2

Here's another way you could do it.
db.collection.aggregate([
{
"$group": {
"_id": "$userId",
"user": {"$first": "$name"},
"damage_dealts": {"$push": "$damage_dealt"},
"maxStat": {"$max": {"$first": "$damage_dealt.stat_value"}}
}
},
{
"$set": {
"outChar": {
"$first": {
"$arrayElemAt": [
"$damage_dealts",
{"$indexOfArray": ["$damage_dealts.stat_value", "$maxStat"]}
]
}
}
}
},
{
"$project": {
"user": 1,
"character_name": "$outChar.character_name",
"stat_value": "$outChar.stat_value"
}
},
{"$sort": {"stat_value": -1}},
{"$limit": 30}
])
Try it on mongoplayground.net.