title: google app engine ndb: put() and then query(), there is always one less item
body: <p>I wonder if anybody has encountered the strange problem on Google App Engine's NDB: after creating a new entity and saving it by <code>put()</code>; and then <code>query()</code> immediately, there is always one less item. For example,</p>
  
  <pre><code>class Item(ndb.Model):
      ...
      ...
  
  items = Item.query().fetch()
  length1 = len(items)
  item = Item()
  item.put()
  items = Item.query().fetch()
  length2 = len(items)
  </code></pre>
  
  <p>In the above, <code>length1</code> is always equal to <code>length2</code>. However, <code>length2</code> will be corrected when revisiting the same HTML page later. What is the problem? Thanks.</p>