angular.module('app', ['firebase'])

  .constant('FBURL', 'https://testbase.firebaseio-demo.com')

  .service('Ref', ['FBURL', Firebase])

  .controller('MyCtrl', MyController)

  .factory('limitMessages', LimitMessagesArray)

  .factory('startAtMessages', StartAtMessagesArray);

​

function LimitMessagesArray(Ref, $q) {

  // return a function so we can alter the limit amount

  return function LimitMessagesArray(lastLimit) {

    var limitQuery = Ref.child('messages').limitToLast(lastLimit);

    // Since it's a one time read we'll use a promise

    var deferred = $q.defer();

    // Reading only once is necessary

    limitQuery.once('value', function(snap) {

      var newMessages = [];

      // iterate through the snapshot and keep the key around

      snap.forEach(function(childSnap) {

        var newMessage = childSnap.val();

        newMessage.$key = childSnap.key();

        newMessages.push(newMessage);

      });

      // resolve the last 'N' messages

      deferred.resolve(newMessages);

    });

    return deferred.promise;

  };

}

​

function StartAtMessagesArray(Ref, $firebaseArray) {

  return function StartAtMessagesArray(startingKey) {

    var rangeQuery = Ref.child('messages').orderByKey().startAt(startingKey);

    return $firebaseArray(rangeQuery);

  };

}

​

function MyController($scope, limitMessages, startAtMessages, Ref) {

  var lastThreeMessages = limitMessages(3);

  // When the last three messages load get the first key

  lastThreeMessages.then(function(data) {

    var startingKey = data[0].$key;

    // Create the sync array of messages from the starting key

    $scope.messages = startAtMessages(startingKey);

  });

}