title: In Google App Engine, how do I use reference properties between two entities that reference each other?
body: <p>If I have two types of models that each reference each other, whichever one I try to define first says it does not recognize the referenced other type (because it is defined further down in the file). For example:</p>
  
  <pre><code>class Author(db.Model):
      most_recent_book = db.ReferenceProperty(Book)
  
  class Book(db.Model):
      author = db.ReferenceProperty(Author)
  </code></pre>
  
  <p>This will claim that the referenced "Book" model is not recognized. If I flip the order, I run into the same issue (except it will say that "Author" is not recognized). How do I get around this? </p>