在谷歌应用引擎中向python类添加更多变量

Question

我有以下代码：

DEFAULT_GUESTBOOK_NAME = 'default_guestbook'

def guestbook_key(guestbook_name=DEFAULT_GUESTBOOK_NAME):
    return ndb.Key('Guestbook', guestbook_name)

class Author(ndb.Model):
    identity = ndb.StringProperty(indexed=False)
    email = ndb.StringProperty(indexed=False)

class Greeting(ndb.Model):
    author = ndb.StructuredProperty(Author)
    content = ndb.StringProperty(indexed=False)
    date = ndb.DateTimeProperty(auto_now_add=True)
    """picture = ndb.BlobProperty(indexed=False)"""

class MapPage(webapp2.RequestHandler):
    def get(self):  
        guestbook_name = self.request.get('guestbook_name',
                                         DEFAULT_GUESTBOOK_NAME)
        greetings_query = Greeting.query(
            ancestor=guestbook_key(guestbook_name)).order(-Greeting.date)
        greetings = greetings_query.fetch(10)

        user = users.get_current_user()

        template_values = {
            'user': user,
            'user_mail': user.email(),
            'greetings': greetings,
            'guestbook_name': urllib.quote_plus(guestbook_name),
            'def_name': DEFAULT_GUESTBOOK_NAME
        }

        path = os.path.join(os.path.dirname(__file__), 'mappage.html')
        self.response.out.write(template.render(path, template_values))

class Guestbook(webapp2.RequestHandler):
    def post(self):
        guestbook_name = self.request.get('guestbook_name',
                                          DEFAULT_GUESTBOOK_NAME)
        greeting = Greeting(parent=guestbook_key(guestbook_name))

        if users.get_current_user():
            greeting.author = Author(
                  identity=users.get_current_user().user_id(),
                  email=users.get_current_user().email())

        greeting.content = self.request.get('content')
        """greeting.picture = self.request.get('picture')"""
        greeting.put()

        query_params = {'guestbook_name': guestbook_name}
        self.redirect('/mappage?' + urllib.urlencode(query_params))

基本上，类Guestbook将Greeting存储到数据存储区中，MapPage会检索它。

但是，每当我在评论中添加代码时，即

picture = ndb.BlobProperty(indexed=False)

到课程问候和

greeting.content = self.request.get('picture')

到类Guestbook，代码无法运行。我已经尝试将图片更改为StringProperty但它仍然无效。