title: Firebase Database Rules are not working in Root/childs with timestamps
body: <p>I have a Firebase Database like this:</p>
  
  <pre><code>"Teilnehmer_Positions" : {
      "user_30OMH3jdfjkd3gd23sdf" : {
        ".priority" : "u1np1kz119",
        "g" : "u1np1kz119",
        "l" : [ 51.8000, 8.50000 ],
        "timestamp" : 1405704395231
      },
      "user_30OMH3jdf3dsdg23sdf" : {
        ".priority" : "u1npct5jq6",
        "g" : "u1npct5jq6",
        "l" : [ 52.0000, 8.40000],
        "timestamp" : 1705704395231
      }
  }
  </code></pre>
  
  <p>I want to read Entrys in which the last update is 10 minutes ago.</p>
  
  <p>Firebase Doku says:</p>
  
  <pre><code>// only messages from the last ten minutes can be read
          ".read": "data.child('timestamp').val() &gt; (now - 600000)",
  </code></pre>
  
  <p>So I build my Rules:</p>
  
  <pre><code>{
      "rules": {
      "Teilnehmer_Positions" : {
          "$users_id" : {
              ".read": "data.child('timestamp').val() &gt; (now - 600000)"
          }
          }
      }
  }
  </code></pre>
  
  <p>But it doesn't work, my query shows nothing.</p>
  
  <p>If I add an timestamp to my Parent it works (and show all of my data):</p>
  
  <pre><code>"Teilnehmer_Positions" : {
      "timestamp" : 1705704395231,
      "user_30OMH3jdfjkd3gd23sdf" : {
        ".priority" : "u1np1kz119",
        "g" : "u1np1kz119",
        "l" : [ 51.8000, 8.50000 ],
        "timestamp" : 1405704395231
      },
      "user_30OMH3jdf3dsdg23sdf" : {
        ".priority" : "u1npct5jq6",
        "g" : "u1npct5jq6",
        "l" : [ 52.0000, 8.40000],
        "timestamp" : 1705704395231
      }
  </code></pre>
  
  <p>}</p>
  
  <p>So I think I have an error in my child root:</p>
  
  <pre><code>"Teilnehmer_Positions" : {
    "$users_id" : {
      ".read": "data.child('timestamp').val() &gt; (now - 600000)"
    }
  }
  </code></pre>
  
  <p>Can anyone help me?</p>