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POIs there a reason that a ReferenceProperty might not generate a back-reference?
 

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  1. POIs there a reason that a ReferenceProperty might not generate a back-reference?
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    <p>In my current project, I have two models, Version and Comment. There is a one-to-many relationship between the two; each Version can have many Comment and the Comment model has a ReferenceProperty to record which Version it belongs to:</p> <pre><code>class Comment(db.Model): version = db.ReferenceProperty(version.Version, collection_name="comments") </code></pre> <p>The problem is that instances of Version are not getting a <em>comments</em> property as I would expect. <a href="http://code.google.com/appengine/docs/python/datastore/typesandpropertyclasses.html#ReferenceProperty" rel="nofollow noreferrer">According to the docs</a>, I should get an automagical property on each Version that is a query that returns all Comment instances that have their version set to the Version instance in question. Doesn't seem to work for my code.</p> <p>I know that the ReferenceProperty is set correctly, because I can get the Comments with this query:</p> <pre><code> comments = comment.Comment.all().filter('version = ', self).order('-added_on').fetch(500) </code></pre> <p>but this does not:</p> <pre><code> comments = self.comments.order('-added_on').fetch(500) </code></pre> <p>it crashes because self has no property comments.</p> <p>The complete code for the two models is included below. Does anyone have any idea why the back-reference property is not given to my Verson instance?</p> <p>from version.py:</p> <pre><code>from google.appengine.ext import db import piece class Version(db.Model): parent_piece = db.ReferenceProperty(piece.Piece, collection_name="versions") note = db.TextProperty() content = db.TextProperty() published_on = db.DateProperty(auto_now_add=True) def add_comment(self, member, content): import comment new_comment = None try: new_comment = comment.Comment() new_comment.version = self new_comment.author = member new_comment.author_moniker = member.moniker new_comment.content = content new_comment.put() except: # TODO: handle datastore exceptions here pass return new_comment def get_comments(self): import comment comments = None try: comments = comment.Comment.all().filter('version = ', self).order('-added_on').fetch(500) except: pass </code></pre> <p>from comment.py:</p> <pre><code>import version import member from google.appengine.ext import db class Comment(db.Model): version = db.ReferenceProperty(version.Version, collection_name="comments") author = db.ReferenceProperty(member.Member) author_moniker = db.StringProperty() author_thumbnail_avatar_url = db.StringProperty() content = db.TextProperty() added_on = db.DateProperty(auto_now_add=True) </code></pre>
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