How to write rule permission for array of objects?

I have a user object which looks like this: { "identities": [{"id": "6a.....", ...}] ... }

iwant to match this field in my flexible sync rule permissions, i have tried this:

{
  "userId": {
    "%stringToOid": "%%user.identities.0.id"
  }
}

It’s not working… Any solution?