Aggregate occurrences of events in nested array - mongodb

Question

Given the following input:
[
{
"statuses": [
{
"status": "allowed",
"count": 3,
"events_count": [
"2001",
"1001",
"1001"
]
}
],
"date": "2022-09-10 15:00",
"_id": "2022-09-10 15:00"
}
]
I need count the number of occurrences of stauses.events_count, so the output would be:
[
{
"statuses": [
{
"status": "allowed",
"count": 3,
"events_count": [
{"type": "2001", "count": 1},
{"type": "1001", "count": 2},
]
}
],
"date": "2022-09-10 15:00",
"_id": "2022-09-10 15:00"
}
]
What I've tried
This is what I got so far:
db.collection.aggregate([
{
"$unwind": "$statuses"
},
{
"$unwind": "$statuses.events_count"
},
{
"$group": {
"_id": {
"event_count": "$statuses.events_count",
"status": "$statuses.status",
"date": "$date",
"count": "$statuses.count"
},
"occurences": {
"$sum": 1
}
}
}
])
Which produces:
[
{
"_id": {
"count": 3,
"date": "2022-09-10 15:00",
"event_count": "2001",
"status": "allowed"
},
"occurences": 1
},
{
"_id": {
"count": 3,
"date": "2022-09-10 15:00",
"event_count": "1001",
"status": "allowed"
},
"occurences": 2
}
]
I'm having difficulties grouping everything back together. I tried grouping by date and pushing back to a 'statuses' array, but it produces two items in the array (with status==allowed), rather than 1 item with status==allowed

Answer 1

You did 2 $unwinds, so it should be 2 $groups in reverse order:
{
"$group": {
"_id": {
"status": "$_id.status",
"count": "$_id.count",
"date": "$_id.date"
},
"event_count": {
"$push": {
"type": "$_id.event_count",
"count": "$occurences"
}
}
}
},
{
"$group": {
"_id": "$_id.date",
"date": {"$last": "$_id.date"},
"statuses": {
"$push": {
"status": "$_id.status",
"count": "$_id.count",
"event_count": "$event_count"
}
}
}
}