MongoDB - Move some fields from the array into another array - mongodb

Question

I have this simplified MongoDB document and would like to change something because there is quite a lot of redundant data. This field "activeUsersLookup" is the result of aggregation which returns data I'd like to put inside the first users array.
First id:
"_id": "80b1565a-faf4-4e68-9bd6-8344060e8d3a" matches
id from activeUsersLookup the same story is with user IDs.
[{
"_id": "80b1565a-faf4-4e68-9bd6-8344060e8d3a",
"users": [
{
"_id": "eaa946da-2708-443e-ab4c-b6db357050ca",
"lastactive": {
"$date": {
"$numberLong": "1637922656000"
}
}
},
{
"_id": "4972ba13-6f4e-4943-be07-15802e22e0dd",
"lastactive": {
"$date": {
"$numberLong": "1653286066000"
}
}
},
{
"_id": "6c4a62ce-c6c6-430f-a0cd-d348ec77dbb2",
"lastactive": {
"$date": {
"$numberLong": "1558623982000"
}
}
}
],
"activeUsersLookup": [
{
"_id": "80b1565a-faf4-4e68-9bd6-8344060e8d3a",
"users": [
{
"_id": "eaa946da-2708-443e-ab4c-b6db357050ca",
"activities": 2
},
{
"_id": "6c4a62ce-c6c6-430f-a0cd-d348ec77dbb2",
"activities": 1
}
],
"sumOfActivities": 3
}
]
}]
So more or less the final document should look like this:
[{
"_id": "80b1565a-faf4-4e68-9bd6-8344060e8d3a",
"users": [
{
"_id": "eaa946da-2708-443e-ab4c-b6db357050ca",
"lastactive": {
"$date": {
"$numberLong": "1637922656000"
}
},
"activities": 2
},
{
"_id": "4972ba13-6f4e-4943-be07-15802e22e0dd",
"lastactive": {
"$date": {
"$numberLong": "1653286066000"
}
},
"activities": 0
},
{
"_id": "6c4a62ce-c6c6-430f-a0cd-d348ec77dbb2",
"lastactive": {
"$date": {
"$numberLong": "1558623982000"
}
},
"activities": 1
},
"sumOfActivities": 3
]
}]
I've tried with:
{
$addFields: {
'licenses.activities': '$activeUsersLookup.users.activities'
}
}
But this gives me an empty array so I must be doing something wrong.
The next stage would be to sum all those activities as sumOfActivities and the last stage would be unset activeUsersLookup.
What magic tricks must I do to have the needed result? :)

Answer 1

I don't think the expected result you posted for the "sumOfActivities": 3 in the users array is valid.
Assume that you are trying to achieve the result as below:
[{
"_id": "80b1565a-faf4-4e68-9bd6-8344060e8d3a",
"users": [...],
"sumOfActivities": 3
}]
The query is a bit long:
$set - Set activeUsersLookup field as object.
1.1. $first - Get the first document from 1.2.
1.2. $filter - Filter document(s) from activeUsersLookup by matching _id for the document in activeUsersLookup with _id (root document).
$set
2.1. - Set users array.
2.1.1. $map - Iterate the documents in users array and return a new array.
2.1.2. $mergeObjects - Merge current documents with the documents with activities field.
2.1.3. $ifNull - Set activities as 0 if no result returned from 2.1.4.
2.1.4. $getField - Get the activities field from the result 2.1.5.
2.1.5. $first - Get the first document from the result 2.1.6.
2.1.6. $filter - Filter the activeUsersLookup.users documents by matching _id for the document (users array) with _id for the current document.
2.2. Set sumOfActivities field.
$unset - Remove activeUsersLookup field.
db.collection.aggregate([
{
$set: {
activeUsersLookup: {
$first: {
$filter: {
input: "$activeUsersLookup",
cond: {
$eq: [
"$$this._id",
"$_id"
]
}
}
}
}
}
},
{
$set: {
users: {
$map: {
input: "$users",
as: "user",
in: {
$mergeObjects: [
"$$user",
{
activities: {
"$ifNull": [
{
"$getField": {
"field": "activities",
"input": {
$first: {
$filter: {
input: "$activeUsersLookup.users",
cond: {
$eq: [
"$$this._id",
"$$user._id"
]
}
}
}
}
}
},
0
]
}
}
]
}
}
},
sumOfActivities: "$activeUsersLookup.sumOfActivities"
}
},
{
$unset: "activeUsersLookup"
}
])
Sample Mongo Playground