Mongodb: Push element in multiple nested array with condition - mongodb

Question

How can I push value into multiple nested array with specific conditions? I have a document like this
[
{
"_id": "class_a",
"students": [
{
"_id": "1a",
"name": "John",
"grades": []
},
{
"_id": "1b",
"name": "Katie",
"grades": []
},
{
"_id": "1c",
"name": "Andy",
"grades": []
},
]
}
]
Query to insert into nested array. (Not sure what is missing here)
db.collection.update({
"_id": "class_a",
"students": {
$elemMatch: {
"_id": {
"$in": [
"1a",
"1b"
]
}
}
}
},
{
$push: {
"students.$.grades": "A+"
}
})
Got the following result. But I was expecting both John and Katie have A+ in grades
[
{
"_id": "class_a",
"students": [
{
"_id": "1a",
"grades": ["A+"],
"name": "John"
},
{
"_id": "1b",
"grades": [],
"name": "Katie"
},
{
"_id": "1c",
"grades": [],
"name": "Andy"
}
]
}
]
Expected result
[
{
"_id": "class_a",
"students": [
{
"_id": "1a",
"grades": ["A+"],
"name": "John"
},
{
"_id": "1b",
"grades": ["A+"],
"name": "Katie"
},
{
"_id": "1c",
"grades": [],
"name": "Andy"
}
]
}
]
Mongo playground to test the code

Answer 1

You can use $[<identifier>] to update only the items that match a condition. Your first {} is to find the relevant documents, while the arrayFilters is to find the relevant items inside the document nested array:
db.collection.update(
{_id: "class_a", students: {$elemMatch: {_id: {$in: ["1a", "1b"]}}}},
{$push: {"students.$[item].grades": "A+"}},
{arrayFilters: [{"item._id": {$in: ["1a", "1b"]}}], upsert: true}
)
See how it works on the playground example

Answer 2

You should really use arrayFilters for these otherwse it'll only match the first entity. You don't need to use $elemMatch at all.
Playground - https://mongoplayground.net/p/_7y89KB83Ho
db.collection.update({
"_id": "class_a"
},
{
$push: {
"students.$[students].grades": "A+"
}
},
{
"arrayFilters": [
{
"students._id": {
"$in": [
"1a",
"1b"
]
}
}
]
})