title: How to set a custom ID when pushing a new object into Firebase
body: <p>I am using Firebase with AngularJS in my application. When a user logs into the app with the Facebook authentication I want to create a new user object (with some properties coming from Facebook and some relative to my app) and store it in Firebase. It's simple, I'm pushing the new User object:</p>
  
  <pre><code>...
  var travelBidsFirebaseRef = new Firebase(url);
  var newUserRef = travelBidsFirebaseRef.child("user").push(user, callback);
  ...
  </code></pre>
  
  <p><code>push()</code> generates a unique ID, smth. like: -J08SgyOeOU_fFb1CB3G. So the only way I can access this User object is by using this ID:</p>
  
  <pre><code>angularFire(travelBidsFirebaseRef + "/user/-J08SgyOeOU_fFb1CB3G", $scope, 'user', {});
  </code></pre>
  
  <p>But when I receive authentication data from Facebook I have only Facebook ID and there is no way of me knowing the Firebase ID generated earlier (otherwise I would have to maintain a map of all users, which I don't want to).
  So here is the question: <strong>how do I save new User to the Firebase with a custom ID, e.g. Facebook ID?</strong></p>