Updating multiple items in nested array when condition matched - mongodb

Question

I started learning MongoDB a few days ago and trying to wrap my head around update operator.
Here's the document structure I have:
db.records.insertMany([
{
name: 'first',
subrecords: [
{ id: 1, isToBeUpdated: 0, value: 1 },
{ id: 2, isToBeUpdated: 0, value: 2 },
{ id: 3, isToBeUpdated: 0, value: 3 },
{ id: 4, isToBeUpdated: 1, value: 4 }
]
},
{
name: 'second',
subrecords: [
{ id: 22, isToBeUpdated: 1, value: 5 },
{ id: 23, isToBeUpdated: 0, value: 6 },
{ id: 24, isToBeUpdated: 1, value: 7 },
{ id: 25, isToBeUpdated: 0, value: 8 }
]
}
])
I'm writing a command that is supposed to update subrecords.
My intent is to use isToBeUpdated flag — if it is set to 1 we update the value field, e.g. multiplying it by 10. We ignore items with isToBeUpdated being 0.
Here's the command I've composed:
db.records.update(
{ 'subrecords.isToBeUpdated': 1 },
{ '$mul': { 'subrecords.$.value': 10 } },
{ multi: true }
)
I expect to get the following items of subrecords array updated like this:
1. { id: 4, isToBeUpdated: 1, value: 40 }
2. { id: 22, isToBeUpdated: 1, value: 50 }
3. { id: 24, isToBeUpdated: 1, value: 70 }
BUT it just updates 2 records, instead of 3. The following record is not updated:
3. { id: 24, isToBeUpdated: 1, value: 70 }
It is the second item with isToBeUpdated being 1 located in the second document's subrecords collection — seems like the query only updates the first matching item of subrecords array in any document. And ignores other records that are subsequent to first.
I tried tweaking my command adding [] to $ (positional operator):
... { '$mul': { 'subrecords.$[].value': 10 } }, ...
But in this case literally every single item in subrecords array of any document gets affected.
Anyone knows how to write a command which loops through all records and updates each item of the nested subrecords array with isToBeUpdated field being set to 1?
Also, am I right in thinking
that { multi: true } implies that ALL record documents in records collection will be affected?
that { 'subrecords.isToBeUpdated': 1 } (query part) literally means that WE ONLY UPDATE subrecords ARRAY WHEN A DOCUMENT HAS AT LEAST ONE ITEM IN subrecords ARRAY WITH isToBeUpdated FIELD BEING SET TO 1?

Answer 1

The workaround turns out to be arrayFilters
https://docs.mongodb.com/manual/reference/operator/update/positional-filtered/#update-all-documents-that-match-arrayfilters-in-an-array
The $[<identifier>] operator facilitates updates to arrays that contain embedded documents. To access the fields in the embedded documents, use the dot notation on the $[<identifier>].
db.records.update(
{},
{ '$mul': { 'subrecords.$[elem].value': 10 } },
{
multi: true,
arrayFilters: [{ 'elem.isToBeUpdated': 1 }]
}
)
Solved!