How to add a field to my projected result that is identical to the argument I sent - mongodb

Question

MY MONGOPLAY
How can I add a field to my result that is identical to the argument I initially sent. Note in my or statement I'm sending a date and I wish to add this date to my results (example requested_by).
I know that '$addFields' would do the trick but couldnt figure out how to integrate it.
My Query:
db.collection.aggregate([
{
"$match": {
"$or": [
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-02-09T00:00:00.000Z") //NEED THIS PARAMETER in my RESULT
}
},
{
"car_id": 79,
"timestamp": {
"$lte": ISODate("2020-03-22T00:00:00.000Z") //NEED THIS PARAMETER in my RESULT
}
}
]
}
},
{
"$sort": {
"timestamp": 1
}
},
{
"$group": {
"_id": "$car_id",
"last": {
"$last": "$$ROOT"
}
}
}
] )
expected result:
[
{
"_id": 78,
"last": {
"_id": ObjectId("60a2c0621e5f043b735e36f0"),
"car_id": 78,
"terminal": "LAX",
"timestamp": ISODate("2020-02-08T17:00:00Z"),
"requested_by": ISODate("2020-02-09T00:00:00.000Z") //<--somethign like this
}
},
{
"_id": 79,
"last": {
"_id": ObjectId("60a2c0621e5f043b735e36f3"),
"car_id": 79,
"terminal": "ORD",
"timestamp": ISODate("2020-03-21T17:00:00Z"),
"requested_by": ISODate("2020-03-22T00:00:00.000Z") //<--somethign like this
}
}
]

Answer 1

You can use '$addFields' in your aggregation as following:
db.collection.aggregate([
{
"$match": {
"$or": [
{
"car_id": 78,
"timestamp": {
"$lte": ISODate("2020-02-09T00:00:00.000Z")
}
},
{
"car_id": 79,
"timestamp": {
"$lte": ISODate("2020-03-22T00:00:00.000Z")
}
}
]
}
},
{
"$sort": {
"timestamp": 1
}
},
{
"$addFields": {
"requested_by": "something"
}
},
{
"$group": {
"_id": "$car_id",
"last": {
"$last": "$$ROOT"
}
}
}
])
If your 'requested_by' field should be calculated based on stored value in the object, you can use pipeline operators to calculate its value. For example:
...
{
"$addFields": {
"requested_by": "$timestamp"
}
},
...
or
...
{
"$addFields": {
"requested_by": {
"$add": [
"$car_id",
20
]
}
}
},
...