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copied!<p><a href="http://code.google.com/appengine/docs/python/datastore/typesandpropertyclasses.html#ReferenceProperty" rel="nofollow">ReferenceProperties</a> add <a href="http://code.google.com/appengine/docs/python/datastore/queryclass.html" rel="nofollow">query-objects</a> as attributes to the referenced class. So look carefully at your mappings:</p> <pre><code>class BookAuthor(db.Model): # This adds a query-object as an attribute named 'books' to Book entities. book = db.ReferenceProperty(Book, required=True, collection_name='books') # This adds a query-object as an attribute named 'authors' to Author entities. author = db.ReferenceProperty(Author, required=True, collection_name='authors') </code></pre> <p>In your code:</p> <pre><code>b = Book(title="My Book") a = Author(name="Author of My Book") </code></pre> <p>So, <code>b</code> would have a <code>books</code> attribute, not <code>authors</code>. And, <code>a</code> would have a <code>authors</code> attribute, not <code>books</code>. </p> <p>If you change the collection names, your code should run.</p> <pre><code>class BookAuthor(db.Model): # This adds a query-object as an attribute named 'authors' to Book entities. book = db.ReferenceProperty(Book, required=True, collection_name='authors') # This adds a query-object as an attribute named 'books' to Author entities. author = db.ReferenceProperty(Author, required=True, collection_name='books') </code></pre> <p>Also, if <code>BookAuthor</code> does not have additional properties, make sure you look at the list-of-keys method outlined in the article you referenced.</p>

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