MongoDB - How to $getField from $$this for $reduce - mongodb

Question

I would like to move an array stored in old_field that looks like this:
[{id: "XXX", ...}, {"id": "YYY", ...}, ...]
Into new_field looking like this:
{"XXX": {id: "XXX", ...}, "YYY":, {id: "YYY", ...}, ...}
As such, I attempted to do a few iterations of the following:
$addFields: {
new_field: {
$reduce: {
input: "$old_field",
initialValue: {},
in: {
{$getField: {field: "id", input: "$$this"}}: "$$this"
}
}
}
}
All of which failed. Note that doing:
$addFields: {
new_field: {
$reduce: {
input: "$old_field",
initialValue: {},
in: {
"1": {$getField: {field: "id", input: "$$this"}}
}
}
}
}
Returns a new_field w/ value 1: {the_correct_id_here}, so I know that the $getField works properly (besides likely using $$this in the wrong context).
Why isn't $getField working in this context? How would I go about doing this transformation?

Answer 1

$addField
1.1. $arrayToObject - Convert the array result from 1.1.1 to object.
1.1.1. $map - Iterate each element in the old_field array and return a new array. Convert each element to an object with k and v properties.
db.collection.aggregate([
{
$addFields: {
new_field: {
$arrayToObject: {
$map: {
input: "$old_field",
in: {
k: "$$this.id",
v: "$$this"
}
}
}
}
}
}
])
Demo # Mongo Playground