title: Google Appengine NDB ancestor vs key query
body: <p>I am storing a key of an entity as a property of another in order to relate them. We are in a refactor stage at this point in the project so I was thinking about introducing ancestors.
  Is there a performance difference between the two approaches? Any given advantages that I might gain if we introduce ancestors?</p>
  
  <pre><code>class Book(ndb.Model):
    ...
  
  class Article(ndb.Model):
    book_key = ndb.KeyProperty(kind=Book, required=True)
  
  
  book_key =  ndb.Key("Book", 12345)
  </code></pre>
  
  <p><strong>1st ancestor query approach</strong></p>
  
  <pre><code>qry = Article.query(ancestor=book_key)
  </code></pre>
  
  <p><strong>2st simple key query approach</strong></p>
  
  <pre><code>qry = Article.query(book_key=book_key)
  </code></pre>