Swap elements in a MongoDB array given only their ids (in-place) - arrays

Question

I read some threads about it, such as this one and that one, but they both come with major flaws & caveats (to me at least).
What I want is simple! An update query that:
swaps 2 elements in an array.
each element is a sub-document.
I am only given the two indexes as input, of those which need to be swapped.
I want to AVOID using a javascript function, if at all possible.
I want it to BE ATOMIC!
Here is the part i'm struggling with:
Option 1: Using update WITH an aggregation pipeline
( summary: succeeds at $arrayElemAt, fails over at "images.0" )
db.users.update(
{ userID: 1 },
[
{ $set: {
"images.0": { $arrayElemAt: ['$images', 2] },
"images.2": { $arrayElemAt: ['$images', 0] },
}}
]
)
If I use an aggregation in my update, I get access to the $arrayElemAt operator, which is great! But this solution fails because this → "images.0" doesn't do what I expect it too. I would have expected it to say "hey, please override the first element inside the images array". But no, what it does is it pushes a new field called "0": { ... }, to each and every sub-document inside the images array.
Option 2: Using update WITHOUT an aggregation pipeline
( summary: succeeds at "images.0", fails over at $arrayElemAt)
db.users.update(
{ userID: 1 },
{ $set: {
"images.0": { $arrayElemAt: ['$images', 2] },
"images.2": { $arrayElemAt: ['$images', 0] },
}}
)
This one fails & succeeds exactly at the opposite places.
The "images.n" DOES replace the n'th element with the given input.
The problem here is, since it is no longer an aggregation, I have no access to the $arrayElemAt operator, and the new n'th element simply becomes:
{
"$arrayElemAt" : [
"$aboutMe.images",
2.0
]
},
Does anyone know what can be done?

Answer 1

This is a nice question that I didn't meet before.
The catch here, is that you need a pipeline to refer existing values, but this prevents working with direct index like dot notation or even $push.
Hence, one option is using $reduce:
$set keys for the wanted values.
$reduce with reference to the size of $$value, according to this answer, in order to replace.
db.users.update(
{userID: 1},
[
{$set: {
firstItem: {$arrayElemAt: ["$images", 2]},
secondItem: {$arrayElemAt: ["$images", 0]}
}
},
{$set: {
images: {
$reduce: {
input: "$images",
initialValue: [],
in: {$concatArrays: [
"$$value",
{$cond: [
{$eq: [{$size: "$$value"}, 0]},
["$firstItem"],
{$cond: [{$eq: [{$size: "$$value"}, 2]},
["$secondItem"],
["$$this"]]
}
]
}
]
}
}
},
firstItem: "$$REMOVE",
secondItem: "$$REMOVE"
}
}
])
See how it works on the playground example