Enrich array of objects with another document based on a condition - mongodb

Question

Given two collections users and activities, I want to enrich users.friends.user with all relevant activities, such that users.friends.user matches doc2.invitations.userId.
Below describes exactly what I would like:
user f24b189f-9e00-4d2b-b7f2-148a265fae7c is not a part of any activity, so their activities array is [].
user 223f2e04-6f0c-40a3-a88a-69d0127f2e92 is a part of an activity, so their activities array is populated with the activity object.
Raw:
db={
"users": [
{
"_id": "9c69a57f-e90a-43be-b559-449e973c6cba",
"__v": {
"$numberInt": "0"
},
"friends": [
{
"status": "ACCEPTED",
"user": "f24b189f-9e00-4d2b-b7f2-148a265fae7c"
},
{
"status": "ACCEPTED",
"user": "223f2e04-6f0c-40a3-a88a-69d0127f2e92"
}
]
}
],
"activities": [
{
"_id": {
"$oid": "63a92531bc6dea668f93bb06"
},
"invitations": [
{
"_id": {
"$oid": "63a92531bc6dea668f93bb07"
},
"userId": "9c69a57f-e90a-43be-b559-449e973c6cba",
"status": "ACCEPTED"
},
{
"_id": {
"$oid": "63a92544bc6dea668f93bb09"
},
"userId": "223f2e04-6f0c-40a3-a88a-69d0127f2e92",
"status": "ACCEPTED"
},
],
"__v": {
"$numberInt": "0"
}
}
]
}
Expected result:
"expectedResult": [
{
"_id": "9c69a57f-e90a-43be-b559-449e973c6cba",
"__v": {
"$numberInt": "0"
},
"friends": [
{
"status": "ACCEPTED",
"activities": [{
"_id": {
"$oid": "63a92531bc6dea668f93bb06"
},
"invitations": [
{
"_id": {
"$oid": "63a92531bc6dea668f93bb07"
},
"userId": "9c69a57f-e90a-43be-b559-449e973c6cba",
"status": "ACCEPTED"
},
{
"_id": {
"$oid": "63a92544bc6dea668f93bb09"
},
"userId": "223f2e04-6f0c-40a3-a88a-69d0127f2e92",
"status": "ACCEPTED"
},
],
"__v": {
"$numberInt": "0"
}
}]
},
{
"status": "ACCEPTED",
"activities": []
}
]
}
]
And here's a link to the playground.
I played around with aggregate and populate, but this use case is unlike a regular "join", which is throwing me off. Any pointers will be appreciated.