title: Flutter: Firebase Real-Time database orderByChild has no impact on query result
body: <p>I have inserted data like this into Firebase Real-Time Database like this way: </p>
  
  <p><a href="https://i.stack.imgur.com/bvVEf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bvVEf.png" alt="enter image description here"></a></p>
  
  <p>And I query the database back like this way: </p>
  
  <pre><code>final databaseReference = FirebaseDatabase.instance.reference();
  databaseReference
      .child('orders')
      .orderByChild('date_slug')
      .limitToFirst(pageSize)
      .once()
      .then((snapshot) {
    firstPageItems = snapshot.value;
  
    if (firstPageItems != null) {
      List&lt;dynamic&gt; curretList = [];
      firstPageItems.forEach((orderId, orderData) {
  
        print ("date_slug " + orderData['date_slug'] + "\r\n\r\n\r\n");
        curretList.add(orderData);
      });
  
      _list.addAll(curretList);
      _listController.sink.add(_list);
    }
  });
  </code></pre>
  
  <p>However, the data didn't come back as sorted as I expected. See the output below. </p>
  
  <pre><code>I/flutter (17125): date_slug 2020-04-20 15:52:13
  I/flutter (17125): 
  I/flutter (17125): 
  I/flutter (17125): date_slug 2020-04-20 15:52:11
  I/flutter (17125): 
  I/flutter (17125): 
  I/flutter (17125): date_slug 2020-04-20 15:52:10
  I/flutter (17125): 
  I/flutter (17125): 
  I/flutter (17125): date_slug 2020-04-20 15:52:12
  </code></pre>