title: Why does this Firebase ".indexOn" not work?
body: <p>So I have data like so: </p>
  
  <pre><code>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
      }
  }
  </code></pre>
  
  <p>In my code, I need to get the latest notification below to a certain "notifyId". Here is the code:</p>
  
  <pre><code>ref.child('pushNotifications')
     .orderByChild("notifyId")
     .limitToLast(1)
     .on("child_added", function (snapshot) {
         console.log("Found a new notification...");
         sendPush(snapshot.val());
     });
  </code></pre>
  
  <p>In my firebase security rule, I have this index rule: </p>
  
  <pre><code> "pushNotifications": {
        ".read": "auth != null",
        ".write": "auth != null",
        "$pushId": {
          ".indexOn": ["notifyId", "timestamp"],
        }
      },
  </code></pre>
  
  <p>It is the index structure follow by the Firebase docs here: <a href="https://www.firebase.com/docs/security/guide/indexing-data.html" rel="noreferrer">docs</a></p>
  
  <p>But when I run the code, the console still log </p>
  
  <blockquote>
    <p>FIREBASE WARNING: Using an unspecified index. Consider adding ".indexOn": "notifyId" at /pushNotifications to your security rules for better performance </p>
  </blockquote>
  
  <p>Which does not make any sense since I did that already. Please help! </p>