Group Subdocuments Array by key in MongoDB - mongodb

Question

Is it possible to use MongoDB's aggregation framework to group channels using folder key and without joining documents?
[{key: 1, channels: {A: [], B: [], etc}}, {key: 2, channels: {A: [], B: [], etc}}]
I am trying to do using $unwind and then $group by folder name but it seems impossible.
Mongo Playground
Documents:
[
{
key: 1,
channels: [
{
"id": 1,
"name": "XXX",
"folder": "C"
},
{
"id": 2,
"name": "XXX",
"folder": "A"
},
{
"id": 3,
"name": "XXX",
"folder": "B"
},
{
"id": 4,
"name": "XXX",
"folder": "A"
},
{
"id": 5,
"name": "XXX",
"folder": "B"
},
{
"id": 6,
"name": "XXX",
"folder": "C"
}
]
},
{
key: 2,
channels: [
{
"id": 1,
"name": "XXX",
"folder": "D"
},
{
"id": 2,
"name": "XXX",
"folder": "B"
},
{
"id": 3,
"name": "XXX",
"folder": "A"
},
{
"id": 4,
"name": "XXX",
"folder": "C"
},
{
"id": 5,
"name": "XXX",
"folder": "A"
},
{
"id": 6,
"name": "XXX",
"folder": "D"
}
]
}
]
Expected Result:
[
{
key: 1,
channels: {
A: [{
"id": 2,
"name": "XXX"
},{
"id": 4,
"name": "XXX"
}],
B: [{
"id": 3,
"name": "XXX"
}, {
"id": 5,
"name": "XXX"
}],
C: [{
"id": 1,
"name": "XXX"
},{
"id": 6,
"name": "XXX"
}]
}
},
{
"key": 2,
"channels": ...
}
]
Thank you very much in advance.

Answer 1

$group by key and folder, construct channels array by providing required fields
$group by only key and construct the channels array in key-vlaue format
$arrayToObject convert above constructed channels to object
db.collection.aggregate([
{ $match: { key: { $in: [1, 2] } } },
{ $unwind: "$channels" },
{
$group: {
_id: {
id: "$key",
folder: "$channels.folder"
},
channels: {
$push: {
id: "$channels.id",
name: "$channels.name"
}
}
}
},
{
$group: {
_id: "$_id.id",
channels: {
$push: {
k: "$_id.folder",
v: "$channels"
}
}
}
},
{ $project: { channels: { $arrayToObject: "$channels" } } }
])
Playground

Answer 2

You just have to add one more $group stage followed by project stage which will alter the data as per requirement.
db.collection.aggregate([
{
$match: {
key: {
$in: [
1,
2
]
}
}
},
{
$unwind: "$channels"
},
{
$group: {
_id: {
key: "$key",
folder: "$channels.folder"
},
value: {
$push: "$channels",
},
"foldersRef": {
$push: "$channels.folder"
},
}
},
{
$group: {
_id: "$_id.key",
"channels": {
"$push": "$value"
},
},
},
{
"$project": {
"channels": {
"$map": {
"input": "$channels",
"as": "channel",
"in": {
"$arrayToObject": [
[
{
"v": {
"$map": {
"input": "$$channel",
"as": "subChannel",
"in": {
"id": "$$subChannel.id",
"name": "$$subChannel.name",
}
}
},
"k": {
"$arrayElemAt": [
"$$channel.folder",
0
]
},
}
]
],
},
}
},
},
},
])
Mongo Playground Sample Execution

Answer 3

What you have done is more appreciated. You might need two grop to easly preform the aggregations
$unwind to deconstruct the array
$group to group by key and foldername and second group to group by key only, but make the grp as key value pair which helps for $arrayToObject
$replaceRoot to make it to root with other fields _id ($mergeobjects)
$project for projection
Here is the code
db.collection.aggregate([
{ $unwind: "$channels" },
{
"$group": {
"_id": { key: "$key", fname: "$channels.folder" },
"channels": { "$push": "$channels" }
}
},
{
"$group": {
"_id": "$_id.key",
"grp": { "$push": { k: "$_id.fname", v: "$channels" } }
}
},
{ "$addFields": { "grp": { "$arrayToObject": "$grp" } } },
{
"$replaceRoot": {
"newRoot": { "$mergeObjects": [ "$grp", "$$ROOT" ] }
}
},
{
"$project": { grp: 0 }
}
])
Working Mongo playground