Mongodb merge update nested fields in a subdocument - node.js

Question

I am searching if it exists, in mongodb or its nodejs driver, any method to merge/update a subdocument like this:
The sample document, for example in collection C:
{
subdocument: {
a: 1,
b: 2
}
}
The update query that I do NOT want to use:
db.C.updateOne(
{},
{
$set: {
"subdocument.b": 3
}
}
}
The update query that I want to use:
db.C.updateOne(
{},
{
$set: {
subdocument: {
b: 3
}
}
}
}
The resulting document I get when running such a query:
{
subdocument: {
b: 3
}
}
The resulting merged document I would like to get instead:
{
subdocument: {
a: 1,
b: 3
}
}
For the record, the reason I want this is because I'm trying to use interfaces to avoid as much as possible writing schema keywords inside strings, so I can have as many of them checked by the typescript compiler as possible. So i know the way to do this is to update "subdocument.b", but I am exactly trying to not do this, to avoid using a string.
Obviously, the subdocument is not merged but fully replaced using the standard update witout options. I would like to know if there is a way to do this natively using mongodb query language, aggregation framework, mongodb nodejs driver, or maybe something else? From what i could learn by myself, it seems unsupported, but maybe it is and someone can tell me how?
Thanks

Answer 1

Its very easy to do that if you use pipeline updates MongoDB >= 4.2
With pipeline updates all aggregate operators can be used, but only limited stages see the database command , drivers use that internally.
If you use pipeline updates you can use only aggregation operators, or query operators for the $match, only if you use $expr.The old update operators doesn't work inside a pipeline.
Test code here
db.collection.update({},
[
{
"$addFields": {
"subdocument": {
"$mergeObjects": [
"$subdocument",
{
"b": 3
}
]
}
}
}
])