如果孩子不存在，如何在Firebase中设置允许写入的规则？

{
  "rules": {
    "users": {
      "$uid": {
        ".write": "!data.exists()",
        ".read": "auth.uid === $uid"
      }
    }
  }
}

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow create: if !exists(/databases/$(database)/documents/users/$(userId));
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}