Update fields of array subdocuments with one request - mongodb

Question

I have the following document in a collection:
{
_id: 'test',
values: [
{ foo: 1, bar: [<very big array>] },
{ foo: 4, bar: [<very big array>] },
{ foo: 3, bar: [<very big array>] }
]
}
I want to update all values[].foo values at once with a pre-calculated array. For performances reasons, I don't want to read the values[].bar arrays and since values can contains many elements, I'm searching a way to do it with only one request (if possible).
For example, I want to write something like this:
db.collection.updateOne({ _id: 'test' }, { $set: { 'values[].foo': [2, 3, 4] }});
And the result would be the following:
{
_id: 'test',
values: [
{ foo: 2, bar: [<very big array>] },
{ foo: 3, bar: [<very big array>] },
{ foo: 4, bar: [<very big array>] }
]
}
But I don't know how i must write my update request.
I'm using MongoDB 4.0 and i don't have access to 4.2 features.

Answer 1

Starting from v4.2 you can benefit from updates with aggregation pipeline.
It gives you ability to calculate new array using $zip and $map:
db.collection.updateOne(
{_id: "test"},
[{$set: {
values: {
$map: {
input: {
$zip: {
inputs: [
"$values.bar",
[2,3,4]
]
}
},
as: "item",
in: {
foo: {
$arrayElemAt: [
"$$item",
1
]
},
bar: {
$arrayElemAt: [
"$$item",
0
]
}
}
}
}
}}]
)
Be sure size of values is the same as size of update array.