Mongodb Update subdocument values matching subdocument and without altering other fields - mongodb

Question

I have a schema like this
{
"_id": "5f86da4b5bb9a62742371409",
"status" : true,
"doc": [
{
"_id": "5f86cacbe4d7c423f21faf50",
"a": 4,
"b": null
},
{
"_id": "5f86cb01a1ca5124063299c1",
"a": 6,
"b": null
}
]
}
And I have an update in the form of
[
{
"_id": "5f86cacbe4d7c423f21faf50",
"b": 90
},
{
"_id": "5f86cb01a1ca5124063299c1",
"b": 45
}
]
How can I update the collection to end up like this?
{
"_id": "5f86da4b5bb9a62742371409",
"status" : true,
"doc": [
{
"_id": "5f86cacbe4d7c423f21faf50",
"a": 4,
"b": 90
},
{
"_id": "5f86cb01a1ca5124063299c1",
"a": 6,
"b": 45
}
]
}
Basically I want to update the subdocument with specific keys only (leaving other keys intact)

Answer 1

You can use update with aggregation pipeline from MongoDB 4.2,
$map to iterate loop of array doc and inside it iterate through updateDocs array, it will return matching b, and then $mergeObjects will return updated document,
let updateDocs = [
{ "_id": mongoose.Types.ObjectId("5f86cacbe4d7c423f21faf50"), "b": 90 },
{ "_id": mongoose.Types.ObjectId("5f86cb01a1ca5124063299c1"), "b": 45 }
];
db.collection.updateMany({},
[
{
$set: {
doc: {
$map: {
input: "$doc",
as: "d",
in: {
$mergeObjects: [
"$$d",
{
$reduce: {
input: updateDocs,
initialValue: {},
in: {
$cond: [
{ $eq: ["$$this._id", "$$d._id"] },
{ b: "$$this.b" },
"$$value"
]
}
}
}
]
}
}
}
}
}
]
)
Playground