Why does this Firebase ".indexOn" not work?

Question

So I have data like so:

pushNotifications {
    -K - RwEgd541PIGNOXXUH: {
        message: "Xiaoyu Hugh Hou bid at $900 on your task: New P..."
        notifyId: "facebook:10100683829966199"
        relatedTask: "-Jzl8XKaxGCv19nIOChm"
        timestamp: 1443594566599
    } - K - RwpcBlQa04VJT4Bwm: {
        message: "Sam Feldt bid at $1100 on your task: New Post g..."
        notifyId: "google:1043268349966197"
        relatedTask: "-Jzl8XKaxGCv19nIOChm"
        timestamp: 1443594721963
    } - K - RwuM3 - 0 G0q9I96WHg: {
        message: "Sam Feldt bid at $600 on your task: NO Firebase..."
        notifyId: "facebook:10100683829966199"
        relatedTask: "-JzTSk2TIlO46oVqJ1Nn"
        timestamp: 1443594741347
    }
}

In my code, I need to get the latest notification below to a certain "notifyId". Here is the code:

ref.child('pushNotifications')
   .orderByChild("notifyId")
   .limitToLast(1)
   .on("child_added", function (snapshot) {
       console.log("Found a new notification...");
       sendPush(snapshot.val());
   });

In my firebase security rule, I have this index rule:

 "pushNotifications": {
      ".read": "auth != null",
      ".write": "auth != null",
      "$pushId": {
        ".indexOn": ["notifyId", "timestamp"],
      }
    },

It is the index structure follow by the Firebase docs here: docs

But when I run the code, the console still log

FIREBASE WARNING: Using an unspecified index. Consider adding ".indexOn": "notifyId" at /pushNotifications to your security rules for better performance

Which does not make any sense since I did that already. Please help!

Answer 1

You need to put the .indexOn one level higher than what you did:

 "pushNotifications": {
    ".read": "auth != null",
    ".write": "auth != null",
    ".indexOn": ["notifyId", "timestamp"]
},

For comparison always look at the Firebase documentation on indexes, which uses this JSON example:

"dinosaurs": {
  "lambeosaurus": {
    "height" : 2.1,
    "length" : 12.5,
    "weight": 5000
  },
  "stegosaurus": {
    "height" : 4,
    "length" : 9,
    "weight" : 2500
  }
}

With this indexing rule:

{
  "rules": {
    "dinosaurs": {
      ".indexOn": ["height", "length"]
    }
  }
}