mongoDB $concat string with array item - mongodb

Question

This is my collection:
[
{
"_id": {
"$oid": "6332dda58121948311cbdb67"
},
"date": "2022-09-13",
"file": "xxx",
"package": 1,
"userList": [
{
"userName": "user_1",
"crDate": "2022.09.28",
"boolId": 1
}
]
},
{
"_id": {
"$oid": "6332dda58121948311cbdb68"
},
"date": "2022-09-13",
"file": "xxx",
"package": 2,
"userList": []
}
]
My desired output would be this (if all of the userList array is empty):
[
{
"_id": "2022-09-13",
"items": [
{
"fileName": "xxx",
"items": [
{
"package": "1 - na"
},
{
"package": "2 - na"
}
]
}
]
}
]
And this if I would have an object inside the userList array:
[
{
"_id": "2022-09-13",
"items": [
{
"fileName": "xxx",
"items": [
{
"package": "1 - user_1"
},
{
"package": "2 - na"
}
]
}
]
}
]
I try to run this aggregate:
db.collection.aggregate([
{
$group: {
_id: {
"date": "$date",
"file": "$file",
},
"items": {
$push: {
"package": {
$concat: [
{
$toString: "package"
},
" - ",
{
$toString: {
$arrayElemAt: [
"$userList",
0
]
}
}
]
}
}
}
}
},
{
$group: {
_id: "$_id.date",
"items": {
$push: {
"fileName": "$_id.file",
"items": "$items"
}
}
},
},
])
It's running if the userList array is empty, but it's not returning the desired output and if the userList array is not empty it throws this error:
Mongo Server error (MongoCommandException): Command failed with error 241 (ConversionFailure): 'Unsupported conversion from object to string in $convert with no onError value' on server
Here comes an example: mongo_playground
How should I modify the aggregate?

Answer 1

It requires 2 fixes in the first $group stage,
missed the $ sign in package property name, in { $toString: "$package" }
use property name userName while accessing first element in $userList.userName
use $ifNull operator to check if the property is not present then it returns "na" string
db.collection.aggregate([
{
$group: {
_id: {
"date": "$date",
"file": "$file"
},
"items": {
$push: {
"package": {
$concat: [
{ $toString: "$package" }, // <====== here
" - ",
{
$toString: {
$ifNull: [
{ $arrayElemAt: ["$userList.userName", 0] }, // <====== here
"na"
]
}
}
]
}
}
}
}
},
{
$group: {
_id: "$_id.date",
"items": {
$push: {
"fileName": "$_id.file",
"items": "$items"
}
}
}
}
])
Playground