MongoDB Lookup Aggregation - mongodb

Question

I have Parking collection like:
parking: {
_id: xxxxxx,
name: xxxxxx,
vehicles: [{vehicleId: xxxxx, bid: xxxxx}...]
}
and car collection:
car: {
_id: "xxxx",
attributes: [xxxxx],
color: "xxxx"
}
When I do Lookup Aggregation:
$lookup: {
from: "car",
localField: "vehicles.vehicleId",
foreignField: "_id",
as: "cars"
}
I get:
parking: {
_id: xxxxxx,
name: xxxxxx,
vehicles: [{vehicleId: xxxxx, bid: xxxxx}],
cars: [car1,car2...]
}
So I struggle with merging new cars array with objects in vehicles array that match id.
Can I somehow replace vehicleId with car document that match?
I tried this but group operation remove name field from parking
db.parking.aggregate([
{ "$unwind": "$vehicles" },
{ "$lookup": {
"from": "car",
"as": "vehicles.vehicle",
"localField": "vehicles.vehicleId",
"foreignField": "_id"
}},
{ "$unwind": "$vehicles.vehicle" },
{ "$group": {
"_id": "$_id",
"vehicles": { "$push": "$vehicles" }
}}
])

Answer 1

It's easier use $map operator by combining the $reduce operator.
Try this one:
db.parking.aggregate([
{
"$lookup": {
"from": "car",
"localField": "vehicles.vehicleId",
"foreignField": "_id",
"as": "cars"
}
},
{
$addFields: {
vehicles: {
$map: {
input: "$vehicles",
as: "veh",
in: {
bid: "$$veh.bid",
vehicleId: {
$reduce: {
input: "$cars",
initialValue: "$$veh.vehicleId",
in: {
$cond: [
{
$eq: [ "$$this._id", "$$veh.vehicleId" ]
},
"$$this",
"$$value"
]
}
}
}
}
}
},
cars: "$$REMOVE"
}
}
])
MongoPlayground | Replace vehicleId