title: Can I get the ID of a related object without accessing the database in Django
body: <p>If I have a instance of an object, can I get the ID of a related object without needing to access the database? The value must be storred somewhere.</p>
  
  <p>eg</p>
  
  <pre><code>book = Book.objects.get(id=1)
  book.author.id
  </code></pre>