Firebase database sort by deeper child

Question

Considering the following structure of the Firebase database:

• root
  • game1
    • $playerUidA
      • score: 50
    • $playerUidB

Answer 1

Although this question is rather old there might be people (like me) stumbling over it. Especially because it is pretty intuitive to structure the database in a similar way the author of the question did, to create (for example) a leaderboard system for a game. Since the answer is a bit outdated I wanted to add some things.

Some time ago the devs added the possibility to order by deeply nested children! (See Ordering by a specified child key) To do so you basically have to do the exact same thing the author did and additionally use the first part of the answer given by @adolfosrs. Basically you have to do two things:

1. Work with .indexOn (as described by @adolfosrs)
2. Use the OrderByChild() command.

To make this work on the example given by @steliosf you would have to do the following:

First set the .indexOn in your database rules:

{
  "rules": {
    "$gameId": {
      ".indexOn": "score",
      "$playerUid": {
        ...  
      }
    }
  }
}

Second use the Command the author of the question already used:

rootRef.Child("root").OrderByChild("score")

I would recommend that you always add a LimitToFirst() or LimitToLast() command to avoid that you pull the whole database which might be a lot of data (depending on the size of your database of course). To get for example the top 10 scores you could use:

rootRef.Child("root").OrderByChild("score").LimitToLast(10)

Since the data is ordered in ascending order you need to use LimitToLast().