Update mongo array elements field from its other field - arrays

Question

Let's assume following document
{
"companies": [
{"id": 1, "name": "Company 1", "new_name": "New company 1"},
{"id": 2, "name": "Company 2", "new_name": "New company 2"}
]
}
Is there a way I can reference element of array by itself to migrate to scheme
{
"companies": [
{"id": 1, "name": "New company 1", "new_name": "New company 1"},
{"id": 2, "name": "New company 2", "new_name": "New company 2"}
]
}
within a single call to update without scripting with forEach in mongo 4.2?
Something like
update(
...
{
"companies.$[].name": "$companies.<???>.new_name"
}
)

Answer 1

To access another field's value in a query you need to use aggregation, taking advantage of executing aggregation pipeline in .update()'s which got introduced in MongoDB version 4.2, try below query :
update(
...
[
{
$addFields: { // will replace existing field with new value
companies: {
$map: { // Iterates on an array & creates a new one
input: "$companies",
in: { $mergeObjects: [ "$$this", { name: "$$this.new_name" } ] } // `mergeObjects` will merge two objects `name` field will be replace in current object i.e; `$$this`
}
}
}
}
]
)
Test : You can test aggregation pipeline here : mongoplayground
Note : Seems to be using aggregation is only option to do this in one call, but downside could be only if your array is too huge, cause $map has to re-write entire array again.