Transform field in document's array - mongodb

Question

I'm trying to convert some fields from string to integer with aggregate pipelines in MongoDB.
These fields are nested in a document's array, so that they are handled as array too.
Below an example document:
{
"_id": *****,
"subject": "first",
"items": [
"item_id": "1",
"ratings": {
"count": 90,
"value": "8",
},
"item_id": "2",
"ratings": {
"count": 75,
"value": "9",
},
]
}
I tried with $convert and $map (on items.item_id and items.rating.value) but I got the following results:
$convert: cannot convert array to int
$map: each field becomes an array itself holding values from all items:
{
"subject": "first",
"items": [
"item_id": [1, 2],
"ratings": {
"count": 90,
"value": [8, 9],
},
"item_id": [1, 2],
"ratings": {
"count": 75,
"value": [8, 9],
},
]
}
Any help is appreciated :)

Answer 1

$map to iterate loop of items array,
$toInt to convert value to integer type
$mergeObjects to merge current object with modified fields value
db.collection.aggregate([
{
$addFields: {
items: {
$map: {
input: "$items",
in: {
$mergeObjects: [
"$$this",
{
"ratings": {
count: "$$this.ratings.count",
value: { $toInt: "$$this.ratings.value" }
}
}
]
}
}
}
}
}
])
Playground