google-cloud-firestore - Firestore Security rules - checking if a string can be cast to an int

Question

There is this page showing how to convert a string to an int:

int("2") == 2
int(2.0) == 2

But how can I know before doing the conversion whether it will work or throw an exception?

For instance how can I implement the following:

IF x can be cast to an integer THEN return int(x) < 10
ELSE IF y can be cast to an integer THEN return int(y) < 10
ELSE return false

标签： google-cloud-firestorefirebase-security

Answer 1

You can use the cast_as_int function defined below:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    function cast_as_int(x) {
      let pattern = '[+-]?([0-9]*[.])?[0-9]+';
      return (
        (x is float)
        || (x is int)
        || (x is string) && (x.matches(pattern))
      ) ? int(float(x)) : null;
    }

    // All gets will succeed
    match /{document=**} {
      allow get: if cast_as_int(1) == 1
        && cast_as_int('2') == 2
        && cast_as_int('3.14') == 3
        && cast_as_int(4.44) == 4
        && cast_as_int("5!") != 5;
    }
  }
}

The function takes in an variable and returns either an Integer or null. Firestore does not allow statements to evaluate to multiple types, so the function can only return Integer or null (not false as requested).

The function assumes you want to convert from (Integer or Float) to Integer. If you only want to convert from Integer to Integer, then replace int(float(x)) with int(x).

The regex for let pattern = '[+-]?([0-9]*[.])?[0-9]+' was taken this StackOverflow Question