Mongo: Upsert to copy all static fields to new document atomically - mongodb

Question

I'm using the Bucket Pattern to limit documents' array size to maxBucketSize elements. Once a document's array elements is full (bucketSize = maxBucketSize), the next update will create a new document with a new array to hold more elements using upsert.
How would you copy the static fields (see below recordType and recordDesc) from the last full bucket with a single call?
Sample document:
// records collection
{
recordId: 12345, // non-unique index
recordType: "someType",
recordDesc: "Some record description.",
elements: [ { a: 1, b: 2 }, { a: 3, b: 4 } ]
bucketsize: 2,
}
Bucket implementation (copies only queried fields):
const maxBucketSize = 2;
db.collection('records').updateOne(
{
recordId: 12345,
bucketSize: { $lt: maxBucketSize } // false
},
{
$push: { elements: { a: 5, b: 6 } },
$inc: { bucketSize: 1 },
$setOnInsert: {
// Should be executed because bucketSize condition is false
// but fields `recordType` and `recordDesc` inaccessible as
// no documents matched and were not included in the query
}
},
{ upsert: true }
)
Possible solution
To make this work, I can always make two calls, findOne() to get static values and then updateOne() where I set fields with setOnInsert, but it's inefficient
How can I modify this as one call with an aggregate? Examining one (last added) document matching recordId (index), evaluate if array is full, and add new document.
Attempt:
// Evaluate last document added
db.collection('records').findOneAndUpdate(
{ recordId: 12345 },
{
$push: { elements: {
$cond: {
if: { $lt: [ '$bucketSize', maxBucketSize ] },
then: { a: 5, b: 6 }, else: null
}
}},
$inc: { bucketSize: {
$cond: {
if: { $lt: [ '$bucketSize', maxBucketSize ] },
then: 1, else: 0
}
}},
$setOnInsert: {
recordType: '$recordType',
recordDesc: '$recordDesc'
}
},
{
sort: { $natural: -1 }, // last document
upsert: true, // Bucket overflow
}
)
This comes back with:
MongoError: Cannot increment with non-numeric argument: { bucketSize: { $cond: { if: { $lt: [ "$bucketSize", 2 ] }, then: 1, else: 0 } }}