MongoDB Aggregation "group" with "max" field within a sub-array - php

Question

Using Compass initially, I then need to convert it into the PHP library.
So far, I have a 1st stage that filters the documents on 2 fields using $match:
comp.id (sub-document / array)
playerId
Code is:
$match (from drop-down)
{
"comp.id" : ObjectId('607019361c071256e4f0d0d5'),
"playerId" : "609d0993906429612483cea0"
}
This returns 2 documents.
The document has a sub-array holes, for the holes played in a round of golf. This sub-array has fields (among others):
holes.no
holes.par
holes.grossScore
holes.nettPoints
So each round has 1 document, with a holes sub-array of (typically) 18 array elements (holes), or 9 for half-round. A player will play each round multiple times - hence multiple documents.
I would like to find the highest holes.nettPoints across the documents. I think I need to $group with $max on the holes.nettPoints field, so I would find the highest score for each hole across all rounds.
I have tried this, but in Compass its says its not properly formatted:
$group drop-down
{
_id: holes.no,
"highest":
{ $max: "$holes.nettPoints" }
}
"highest" can be any name I want?
EDIT FOLLOWING PROVIDED ANSWER
The answer marked as the solution was enough of a pointer for how the Aggregation Framework operates (multi-stage documents, i.e. documents as input to 1 stage become new documents as the output of that stage. And so on.
For the purposes of posterity, I ended up using the following aggregation:
[{$match: {
"comp.id" : ObjectId('607019361c071256e4f0d0d5'),
"playerId" : "609d0993906429612483cea0",
"comp.courseId" : "608955aaabebbd503ba6e116"
}
}, {$unwind: {
path : "$holes"
}}, {$group: {
_id: "$holes.no",
hole: {
$max: "$holes"
}
}}, {$sort: {
"hole": 1
}}]
In PHP speak, it looks like:
$match = [
'$match' => [
'comp.id' => new MongoDB\BSON\ObjectID( $compId ),
'playerId' => $playerId,
'comp.courseId' => $courseId
]
];
$unwind = [
'$unwind' => [
'path' => '$holes'
]
];
$group = [
'$group' => [
'_id' => '$holes.no',
'hole' => [
'$max' => '$holes'
]
]
];
$sort = [
'$sort' => [
'hole.no' => 1
]
];
$cursor = $collection->aggregate([$match, $unwind, $group, $sort]);
It is not complete (looking at adding a $sum accumulator across the courseId, not individual documents), but answers the question posted.

Answer 1

$match your conditions
$unwind deconstruct holes array
$sort by nettPoints in descending order
$group by no and select first holes object
[
{
$match: {
"comp.id": ObjectId("607019361c071256e4f0d0d5"),
"playerId": "609d0993906429612483cea0"
}
},
{ $unwind: "$holes" },
{ $sort: { "holes.nettPoints": -1 } },
{
$group: {
_id: "$holes.no",
highest: { $first: "$holes" }
}
}
]