title: generate custom key with $add angularfire
body: <p>How can I generate a custom key in array inside angularfire when I add a new record with the <code>$add</code> function. Just look at when the is the comment below in the source code. This is my code below but still get the random key generated by firebase.</p>
  
  <pre class="lang-js prettyprint-override"><code>register : function(user){
    return simpleLogin.$createUser({
      email: user.email,
      password: user.password
    }).then(function(regUser){
      //console.dir(regUser);
      var ref = new Firebase(FIREBASE_URL + 'users');
      var firebaseUsers = $firebaseArray(ref);
      var userInfo = {
        date: Firebase.ServerValue.TIMESTAMP,
        regUser: regUser.uid,
        firstname: user.firstname,
        lastname: user.lastname,
        email: user.email
      }
  
      //this is when i want to generate the key
      firebaseUsers.$add(userInfo).then(function(ref) {
  
      });
    });
  },//register
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ACTT9.jpg" alt="still got the random key with the $add function"></p>