function openDatabase() {
  // If the browser doesn't support service worker,
  // we don't care about having a database
  if (!navigator.serviceWorker) {
    return Promise.resolve();
  }

  // TODO: return a promise for a database called 'wittr'
  // that contains one objectStore: 'wittrs'
  // that uses 'id' as its key
  // and has an index called 'by-date', which is sorted
  // by the 'time' property
  return idb.open('wittr', 1, function(upgradeDb) {
    switch(upgradeDb.oldVersion) {
      case 0:
        var wittrStore = upgradeDb.createObjectStore('wittrs', { keyPath: 'id' });
        wittrStore.createIndex("by-date", 'time');
    }
  });
}

// called when the web socket sends message data
IndexController.prototype._onSocketMessage = function(data) {
  var messages = JSON.parse(data);

  this._dbPromise.then(function(db) {
    if (!db) return;

    // TODO: put each message into the 'wittrs'
    // object store.
    var tx = db.transaction('wittr', 'readwrite');
    var wittrStore = tx.objectStore('wittrs');
    messages.forEach(function(msg) {
      wittrStore.put(msg);
    });
    return tx.complete;
  }).then(function() {
    console.log('New messages added!');
  });

  this._postsView.addPosts(messages);
};