mongodb aggregation with multidimensional array - mongodb

Question

How can i aggregate two collections if i have the index in multidimensional array?
//collection 1 item example:
{
_id: ObjectId('000'),
trips: [ // first array
{ // trip 1
details: [ // array in first array
{ // detail 1
key: ObjectId('111') // ==> _id i want aggregate
}
]
}
]
}
//collection 2 item exampe:
{
_id: ObjectId('111'),
description: 'any info here...'
}
what is the procedure to do in order to obtain a valid result?
I don't want to reprocess every data after doing a "find".
This is the result I would like to get
//collection 1 after aggregate:
{
_id: ObjectId('000'),
trips: [
{ // trip 1
details: [
{ // detail 1
key: ObjectId('111') // id i want aggregate
_description_obj: {
_id: ObjectId('111'),
description: 'any info here...'
}
}
]
}
]
}

Answer 1

Here's one way you could do it.
db.coll1.aggregate([
{
"$match": {
_id: ObjectId("000000000000000000000000")
}
},
{
"$lookup": {
"from": "coll2",
"localField": "trips.details.key",
"foreignField": "_id",
"as": "coll2"
}
},
{
"$set": {
"trips": {
"$map": {
"input": "$trips",
"as": "trip",
"in": {
"$mergeObjects": [
"$$trip",
{
"details": {
"$map": {
"input": "$$trip.details",
"as": "detail",
"in": {
"$mergeObjects": [
"$$detail",
{
"_description_obj": {
"$first": {
"$filter": {
"input": "$coll2",
"cond": {"$eq": ["$$this._id", "$$detail.key"]}
}
}
}
}
]
}
}
}
}
]
}
}
},
"coll2": "$$REMOVE"
}
}
])
Try it on mongoplayground.net.