title: How create a django model to filter by a foreign key field
body: <p>I have two models which are connected by a foreign key.</p>
  
  <pre><code>Book
   isbn - this is a unique value
  
  Order
   book_isbn - this should refer to Book's isbn above.
  </code></pre>
  
  <ol>
  <li>How to introduce Order model the isbn of the Book as a foreign key.</li>
  <li>I want to query orders which have given isbn (i.e. filter Order objects where <strong>book_isbn = given_isbn</strong> ?</li>
  </ol>