Or statement returns the first result only (should be 2) - mongodb

Question

I have an OR statement where the expected result should return 2 documents but the return result is only 1 document. I have a list of cars docs:
[
{
"_id": ObjectId("60a2c0621e5f043b735e36ef"),
"car_id": 78,
"terminal": "JFK",
"timestamp": ISODate("2020-01-01T17:00:00.000Z"),
},
{
"_id": ObjectId("60a2c0621e5f043b735e36f0"),
"car_id": 78,
"terminal": "LAX",
"timestamp": ISODate("2020-02-08T17:00:00.000Z"),
},
{
"_id": ObjectId("60a2c0621e5f043b735e36f1"),
"car_id": 78,
"terminal": "ORD",
"timestamp": ISODate("2020-03-01T17:00:00.000Z"),
},
]
and my query asks for 2 instances with SAME CAR ID but with DIFFERENT timestamp:
db.collection.aggregate([
{
"$match": {
"$or": [
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-02-15T05:00:11.000Z")
}
},
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-03-02T11:07:27.000Z")
}
}
]
}
},
{
"$sort": {
"timestamp": 1
}
},
{
"$group": {
"_id": "$car_id",
"last": {
"$last": "$$ROOT"
}
}
}
])
So I expect to get 2 results (for each request).
Expected result:
[
{
"_id": 78,
"last": {
"_id": ObjectId("60a2c0621e5f043b735e36f1"),
"car_id": 78,
"terminal": "ORD",
"timestamp": ISODate("2020-03-01T17:00:00Z")
}
},
{
"_id": 78,
"last": {
"_id": ObjectId("60a2c0621e5f043b735e36f0"),
"car_id": 78,
"terminal": "LAX",
"timestamp": ISODate("2020-02-08T17:00:00Z")
}
}
]
But I only get the first result. How can I get the desired 2 results?
mongoplayground

Answer 1

Because all of the returned docs has the same car_id and you use car_id in $group stage so they will be grouped into one result. To get your expected result, you can add a field depend on timestamp of the doc then use that filed in $group:
db.collection.aggregate([
{
"$match": {
"$or": [
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-02-15T05:00:11.000Z")
}
},
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-03-02T11:07:27.000Z")
}
}
]
}
},
{
"$sort": {
"timestamp": 1
}
},
{
$addFields: {
group: {
$cond: {
if: {
$lte: [
"$timestamp",
ISODate("2020-02-15T05:00:11.000Z")
]
},
then: 1,
else: 2
}
}
}
},
{
"$group": {
"_id": "$group",
"last": {
"$last": "$$ROOT"
}
}
}
])
Mongoplayground
Note: If you have more than one car_id in the result, you can change the $group to:
{
"$group": {
"_id": {
car_id: "$car_id",
group: "$group",
},
"last": {
"$last": "$$ROOT"
}
}
}
Mongoplayground