title: Is it possible to access children of $location variables in Firebase rules?
body: <p>Having this data structure:</p>
  
  <pre><code>users:{
      AYhiJQchwKSwG5p0epd1GrZAQMg2: {
        savedPhotos:{
          21142:true
          123:true
        }
        .
        .
      }
  photos:{
      0:{
        id:9213
        install_count:4
      }
      .
      .
   }
  </code></pre>
  
  <p>I would like to restrict users to save a photo more than once.
  Something like this:
  ...</p>
  
  <pre><code>"install_count":{
            ".write":"root.child('users/'+auth.uid).exists()" ,
            ".validate":"root.child('users/savedPhotos'+$photo.id).exists()"
          }
  </code></pre>
  
  <p>...</p>