Q:

How to write .indexOn for dynamic keys in firebase?

I have data set up like this:
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"
    }
  }
}

I reference it by date so I only get the tasks made after a certain time.
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());
});

This all works fine, for each group I get a warning message saying something like this: 
FIREBASE WARNING: Using an unspecified index.  Consider adding ".indexOn": "date" at tasks/Group 1 to your security rules for better performance

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?

A:

I guess you're looking for:
{
  ".read": true,
  ".write": true,
  "rules": {
    "Tasks": {
      "$groupid": {
        ".indexOn": "date"
      }
    }
  }
}

The Firebase documentation is usually a pretty good read. Spending an hour or less in the security guide, will ensure you have a better understanding of this and many other topics.

