$addField with subQuery in MongoDB aggregation - mongodb

Question

I have 2 collections: Chargers and Reservation. I would like for the reservations in the next 7 days to be appended as a field when querying for chargers (i.e. time range from now until the same day next week).
The model for chargers looks like this:
{
"name":"charger 2",
"address":"test, test, USA",
"current_type": 1,
"charge_level" : 2,
"plug_type" : 1,
}
and the reservation model looks like this:
{
"charger_id": ObjectId("test ID"),
"start_time": "31-01-2021",
"end_time": "1-02-2021"
}
I found the aggregation $addField that adds a field within an aggregation pipeline, but I was wondering if I could use the $addfield on a "subquery". Essentiallly, for each charger that match, get all the reservations within a given time range where the chargerID is the one from the match, and add the array of reservations as a field. The resulting model would look like this:
{
"name":"charger 2",
"address":"test, test, USA",
"current_type": 1,
"charge_level" : 2,
"plug_type" : 1,
"reservations" : [
...
]
}
The current way of getting the data is with 2 queries at the application result, but that becomes quite taxing with the network latencies.
Query 1 - chargers:
{"_id":<charger_id>}
once I get the result, I query again.
Query 2 - Reservations:
{
"charger_id": <charger_id>,
"start_time" : {
"$lte" : <date in a week>
}
}
This is not too bad with a get by ID, because I already have the ID before querying for the charger in the first place, but with a getAll, or any other query that doesn't query by ID, it can get really taxing.

Answer 1

To bring in data from another collection you need to use $lookup. $addFields can be used to add fields based on data you already have in the result set.