MongoDB - Convert nested array values

I have the following structure (subset):

{"values"=>
  [{"foo"=>"12"},
   {"foo"=>"34"},
   {"foo"=>"56"},...]}

and I want to convert the fields from string to decimal like this

db.foos.updateMany(
      { "values" => { "$type": "array" } },
      { "$set": { "values.$[element].foo": { "$toDecimal": "$values.$[element].foo" } } },
      arrayFilters: [{ "element.foo": { "$type": "string" } }]
    )

but I'm getting a literal { "$toDecimal": "$values.$[element].foo" } } written to the field. (I need the string filter to be able to add a $trim later (something like { '$toDecimal': { '$trim': { input: "$foo" } } }))

What am I doing wrong?

In MongoDB update query, you can't update value by referencing the field name.

You should look for update with aggregation pipeline.

db.foos.update({
  "values": {
    "$type": "array"
  }
},
[
  {
    "$set": {
      "values": {
        "$map": {
          "input": "$values",
          "in": {
            "$cond": {
              "if": {
                "$eq": [
                  {
                    "$type": "$$this.foo"
                  },
                  "string"
                ]
              },
              "then": {
                "$mergeObjects": [
                  "$$this",
                  {
                    "foo": {
                      "$toDecimal": {
                        "$trim": {
                          "input": "$$this.foo"
                        }
                      }
                    }
                  }
                ]
              },
              "else": "$$this"
            }
          }
        }
      }
    }
  }
])

Demo @ MongoPlayground

For aggregate query:

1. $match - Filter document
2. $set - Set values field. Iterate element in values array and transform foo value to Decimal.
3. $merge - Merge the output of the aggregation pipeline to collection by _id.

db.foos.aggregate([
  {
    "$match": {
      "$expr": {
        "$eq": [
          {
            "$type": "$values"
          },
          "array"
        ]
      }
    }
  },
  {
    "$set": {
      "values": {
        "$map": {
          "input": "$values",
          "in": {
            "$cond": {
              "if": {
                "$eq": [
                  {
                    "$type": "$$this.foo"
                  },
                  "string"
                ]
              },
              "then": {
                "$mergeObjects": [
                  "$$this",
                  {
                    "foo": {
                      "$toDecimal": {
                        "$trim": {
                          "input": "$$this.foo"
                        }
                      }
                    }
                  }
                ]
              },
              "else": "$$this"
            }
          }
        }
      }
    }
  },
  {
    "$merge": {
      "into": "foos",
      "on": "_id",
      "whenMatched": "replace"
    }
  }
])

Demo @ Mongo Playground