MongoDB multiple nested groups - mongodb

Question

I have documents in mongodb like this
{
_id: "5cfed55974c7c52ecc33ada8",
name: "Garona",
realm: "Blackrock",
faction: "Horde",
race: "Orc",
class: "Rogue",
guild: "",
level: 33,
lastSeen: "2019-06-10T00:00:00.000Z",
__v: 0
},
{
_id: "5cfed55974c7c52ecc33ade8",
name: "Muradin",
realm: "Alleria",
faction: "Alliance",
race: "Dwarf",
class: "Warrior",
guild: "Stormstout Brewing Co",
level: 42,
lastSeen: "2019-06-11T00:00:00.000Z",
__v: 0
}
What I'm trying to do, is to group by a fields and get a sum of it. So far I figured it out to do it for one field at once like so
{
$group: {
_id: {
classes: '1',
class: '$class'
},
total: { $sum: 1 }
}
},
{
$group: {
_id: '$_id.classes',
total: { $sum: '$total' },
classes: {
$push: {
class: '$_id.class',
total: '$total'
}
}
}
}
Which produces something like this
{
_id: "1",
total: 40,
classes: [
{
class: "Warrior",
total: 17
},
{
class: "Rogue",
total: 23
}
}
But I want to do it for more than one field at once, so that I can get an output like this.
{
_id: "1",
total: 40,
classes: [
{
class: "Warrior",
total: 17
},
{
class: "Rogue",
total: 23
},
factions: [
{
faction: "Alliance",
total: 27
},
{
faction: "Horde",
total: 13
}
}
No I'm wondering if it is even possible to do it in one query in an easy way or if I would be better to do a seperate query for each field.

Answer 1

You can do this by using the $facet aggregation stage
Processes multiple aggregation pipelines within a single stage on the same set of input documents. Each sub-pipeline has its own field in the output document where its results are stored as an array of documents.
I only slightly modified your original pipeline, and then just copied it for the 'factions' field.
The last 3 stages in my solution aren't really necessary, they just clean up the output a little bit.
You can probably take it from here, good luck.
db.collection.aggregate([
{
"$facet": {
"classes": [
{
$group: {
_id: "$class",
total: {
$sum: 1
}
}
},
{
$group: {
_id: null,
total: {
$sum: "$total"
},
"classes": {
$push: {
class: "$_id",
total: "$total"
}
}
}
}
],
"factions": [
{
$group: {
_id: "$faction",
total: {
$sum: 1
}
}
},
{
$group: {
_id: null,
total: {
$sum: "$total"
},
"factions": {
$push: {
faction: "$_id",
total: "$total"
}
}
}
}
]
}
},
{
$unwind: "$classes"
},
{
$unwind: "$factions"
},
{
$project: {
"classes._id": 0,
"factions._id": 0
}
}
])
Output
[
{
"classes": {
"classes": [
{
"class": "Warrior",
"total": 1
},
{
"class": "Rogue",
"total": 1
}
],
"total": 2
},
"factions": {
"factions": [
{
"faction": "Alliance",
"total": 1
},
{
"faction": "Horde",
"total": 1
}
],
"total": 2
}
}
]