Query only for numbers in nested array - arrays

Question

I am trying to get an average number of an key in a nested array inside a document, but not sure how to accomplish this.
Here is how my document looks like:
{
"_id": {
"$oid": "XXXXXXXXXXXXXXXXX"
},
"data": {
"type": "PlayerRoundData",
"playerId": "XXXXXXXXXXXXX",
"groupId": "XXXXXXXXXXXXXX",
"holeScores": [
{
"type": "RoundHoleData",
"points": 2
},
{
"type": "RoundHoleData",
"points": 13
},
{
"type": "RoundHoleData",
"points": 3
},
{
"type": "RoundHoleData",
"points": 1
},
{
"type": "RoundHoleData",
"points": 21
}
]
}
}
Now, the tricky part of this is that I only want the average of points for holeScores[0] of all documents with this playerid and this groupid.
Actually, the best solution would be collecting all documents with playerid and groupid and create a new array with the average of holeScores[0], holeScores[1], holeScores[2]... But if I only can get one array key at the time, that would be OK to :-)
Here is what I am thinking but not quit sure how to put it together:
var allScores = dbCollection('scores').aggregate(
{$match: {"data.groupId": groupId, "playerId": playerId}},
{$group: {
_id: playerId,
rounds: { $sum: 1 }
result: { $sum: "$data.scoreTotals.points" }
}}
);
Really hoping for help with this issue and thanks in advance :-)

Answer 1

You can use $unwind with includeArrayIndex to get index and then use $group to group by that index
dbCollection('scores').aggregate(
{
$match: { "data.playerId": "XXXXXXXXXXXXX", "data.groupId": "XXXXXXXXXXXXXX" }
},
{
$unwind: {
path: "$data.holeScores",
includeArrayIndex: "index"
}
},
{
$group: {
_id: "$index",
playerId: { $first: "data.playerId" },
avg: { $avg: "$data.holeScores.points" }
}
}
)

Answer 2

You can try below aggregation
db.collection.aggregate(
{ "$match": { "data.groupId": groupId, "data.playerId": playerId }},
{ "$group": {
"_id": null,
"result": {
"$sum": {
"$arrayElemAt": [
"$data.holeScores.points",
0
]
}
}
}}
)