title: mykey.get() method doesn't work in google app engine project
body: <p>I am passing one of my projects to the google app engine, just for the sake of learning. However I have some problems with the ndb datastore. My root entity would be Categories and these have Subcategories as child entities. So let's say I have Category <em>Foods</em> which has Subcategory <em>Main Dishes</em>. So the key for this Entity would be <code>ndb.Key(Category, 'Foods', Subcategory, subcategoryId)</code>. When I am creating a <strong>query object</strong> from this key I can <strong>fetch</strong> the correct subcategory, but from the documentation I would like to do other two methods as well which are not working, I don't know for what reason.</p>
  
  <p><code>mySubcategorykey.get()</code> => it returns None using the aforementioned key.
  <code>Subcategory.get_by_id(subcategoryId)</code> => Also returns None.</p>
  
  <p>Also when I am generating a safeUrl from the key, I cannot return the object with <code>ndb.Key(urlSafe=myUrlSafeString).get()</code>, however printing the <code>ndb.Key(urlSafe)</code> gives me the correct key, as it states in the <strong>DataStore Viewer</strong>.</p>
  
  <p>Anyone can help me please telling what I am doing wrong? Thank you.</p>