Mongodb Aggregate using $group twice - mongodb

Question

I have a bunch of documents in mongo with the following structure:
{
"_id" : "",
"number" : 2,
"colour" : {
"_id" : "",
"name" : "Green",
"hex" : "00ff00"
},
"position" : {
"_id" : "",
"name" : "Defence",
"type" : "position"
},
"ageGroup" : {
"_id" : "",
"name" : "Minor Peewee",
"type" : "age"
},
"companyId" : ""
}
I'm currently using Mongo's aggregate to group the documents by ageGroup.name which returns:
//Query
Jerseys.aggregate([
{$match: { companyId: { $in: companyId } } },
{$group: {_id: "$ageGroup.name", jerseys: { $push: "$$ROOT" }} }
]);
//returns
{
_id: "Minor Peewee",
jerseys: array[]
}
but I'd like it to also group by position.name within the age groups. ie:
{
_id: "Minor Peewee",
positions: array[]
}
//in positions array...
{
_id: "Defence",
jerseys: array[]
}
// or ageGroups->positions->jerseys if that makes more sense.
I've tried multiple groups but I don't think I'm setting them up correctly I always seem to get an array of _id's. I'm using Meteor as the server and I'm doing it within a meteor method.

Answer 1

You can use a composite aggregate _id in the first grouping stage.
Then, you can use one of those keys as the "main" _id of the final aggregate and $push the other into another array.
Jerseys.aggregate([
{
$match: { companyId: { $in: companyId } }
},
{
$group: { // each position and age group have an array of jerseys
_id: { position: "$position", ageGroup: "$ageGroup" },
jerseys: { $push: "$$ROOT" }
}
},
{
$group: { // for each age group, create an array of positions
_id: { ageGroup: "$_id.ageGroup" },
positions: { $push: { position: "$_id.position", jerseys:"$jerseys" } }
}
}
]);