firestore security rules / check field type in nested object

Question

How do I use belows generic function for nested fields? I have this document below which lives at /userdata/{uid}:

As can be seen userBirthTime is a map, it contains another map called birthTime which in turn has the fields I want to veryify for their types e.g. check that the incoming year is of type int before it's allowed to be stored in firestore.

How would I do this? Is it even possible for nested fields? I tried with

function isInteger(fieldName) { return request.resource.data[fieldName] is int }

and then used it like this

isInteger('userBirthTime.birthTime.year')

with the overall security rule being this

match /userdata/{uid} {
  allow write: if isInteger('userBirthTime.birthTime.year')
}

but that's just always returning true even if I try with a string as fieldName rather than the required int type?!

What am I doing wrong? Is it the fact that's a nested field and not a top-level one?

Answer 1

Apparently the dot notation doesn't work in security rules (although it returns false always):

// the request body
"data": {
  "userBirthTime": {
    "birthTime": {
      "year": "string"
    }
  }
}

Changing the rules to this works for me:

function isInteger(fieldName) { 
  return request.resource.data.userBirthTime.birthTime[fieldName] is int 
}

match /userdata/{uid} {
  allow write:  if isInteger('year'); // other fields
}