title: How to write .indexOn for dynamic keys in firebase?
body: <p>I have data set up like this:</p>
  
  <pre><code>Tasks:{
    Group 1:{
      pushid1:{
        date: 18921949021,
        title: "Do something"
      },
      pushid2:{
        date: 18921949021,
        title: "Do something else"
      }
    },
    Group 2:{
      pushid3:{
        date: 18921949021,
        title: "Do another thing"
      }
    }
  }
  </code></pre>
  
  <p>I reference it by date so I only get the tasks made after a certain time.</p>
  
  <pre><code>var now = Date.now();
  var taskRef = new Firebase(FB + "/tasks/" + thisGroup);
  taskRef.orderByChild('date').startAt(now).on("child_added", function(snap){
    console.log(snap.val());
  });
  </code></pre>
  
  <p>This all works fine, for each group I get a warning message saying something like this: </p>
  
  <pre><code>FIREBASE WARNING: Using an unspecified index.  Consider adding ".indexOn": "date" at tasks/Group 1 to your security rules for better performance
  </code></pre>
  
  <p>How do I add that to my security rules?  I can't add one for each group a user makes, is there a wild card entry of some type?</p>