title: AngularFire return last sorted record by custom key value
body: <p>I have a collection of data in Firebase that uses my own custom keys to match what we are expecting from our API when it is completed.  However, I need to get the highest value from a key that we created.</p>
  
  <p>Data:</p>
  
  <pre><code>-Results
      +20150514-1
      +20150514-2
      -20150521-1
           Field1: data
           Field2: data
           ...
      +20150521-2
      +20150528-1
      +20150528-2
      +20150528-3
      +20150528-4
      +20150604-1
      +20150604-2
      +20150604-3
  </code></pre>
  
  <p>In this case the key fields under my Results are dates with instances, then full records under each instance (Field1, Field2, ...).  I am getting the data using </p>
  
  <pre><code>    var FireBaseData = new Firebase(FIREBASE.URL);
      var Results = FireBaseData.child('Results');
      var LastResult = Results.orderByPriority().limitToLast(1);
      var LastDate = LastResult.$id;
      return LastDate.substr(0, 8);
  </code></pre>
  
  <p>However, I am now trying to get access to the $id, or even a data field in that last record, but I cannot get data.</p>
  
  <p>I have also tried:</p>
  
  <pre><code>factory('Result' ...
      var FireBaseData = new Firebase(FIREBASE.URL);
      var Results = FireBaseData.child('Results');
      var LastResult = Results.orderByPriority().limitToLast(1);
      return $firebaseObject(LastDate);
  </code></pre>
  
  <p>and then try to access the Field on the returned item, without success.</p>
  
  <pre><code>var myDate = Result.Field2;
  </code></pre>