Mongo $lookup aggregation using date timestamps - mongodb

Question

I have two collections: Car Drive Histories and Car Geolocations. For the purpose of analyzing drive patterns I have to aggregate driving histories and link them to car geolocations.
I've used $match and $project aggregation stages to get drive history documents with the following structure:
travelPurpose:<String>
carID:<ObjectId>
checkOutTime:<Date>
checkInTime:<Date>
The next step is to use $lookup stage to get car location between the two timestamps (checkOutTime and checkInTime). Every car geolocation document has carID and geoLocationTimestamp fields. If I use static dates, for example as such:
{
from: 'carGeoLocations',
localField: 'carID',
foreignField: 'carID',
pipeline: [
{$match: {
geoLocationTimestamp: {
$gte: ISODate('2022-01-01T00:00:00.000+0000'),
$lte: ISODate('2023-01-01T00:00:00.000+0000')
}
}}
],
as: 'coordinates'
}
I do get geolocations between 1. 1. 2022 and 1. 1. 2023. Mongo Playground with an example of this behaviour can be accessed here.
However, if I try to use dynamic dates based on values of checkOutTime and checkInTime, no documents are retrieved. Mongo playground with this example is available here. I've tried the following:
{
from: 'carGeoLocations',
localField: 'carID',
foreignField: 'carID',
pipeline: [
{$match: {
geoLocationTimestamp: {
$gte: "$checkOutTime",
$lte: "$checkInTime"
}
}}
],
as: 'coordinates'
}
and
{
from: 'carGeoLocations',
localField: 'carID',
foreignField: 'carID',
let: {t1: '$checkOutTime', t2: '$checkInTime'}
pipeline: [
{$match: {
geoLocationTimestamp: {
$gte: '$$t1',
$lte: '$$t2'
}
}}
],
as: 'coordinates'
}
With the same results. Can anyone spot any issues with my approach?

Answer 1

First lookup and than use match for geoLocationTimestamp
Try following code
{
$lookup:{
from: 'carGeoLocations',
localField: 'carID',
foreignField: 'carID',
as: 'coordinates'
}
},
{
$match:{
geoLocationTimestamp: {
$gte:'$coordinates.checkOutTime',
$lte:'$coordinates.checkInTime'
}
}
}

Answer 2

Update
After further experimentation, it turns out you need to use $expr when you want to use variables declared with let in $lookup stage of aggretation.
My lookup stage now looks like this:
{
"$lookup": {
"from": "carGeoLocations",
"localField": "carID",
"foreignField": "carID",
"let": {
t1: "$checkOutTime",
t2: "$checkInTime"
},
"pipeline": [
{
$match: {
$and: [
{
$expr: {
$gte: [
"$geoLocationTimestamp",
"$$t1"
],
}
},
{
$expr: {
$lte: [
"$geoLocationTimestamp",
"$$t2"
],
}
},
]
}
}
],
"as": "coordinates"
}
}