i using google-app-engine webapp ,code is :

class Post(db.Model):
    title = db.StringProperty(required=True)
    def categories(self):
        return (x.category for x in self.postcategory_set)

class Category(db.Model):
    name = db.StringProperty()

class PostCategory(db.Model):
    post = db.ReferenceProperty(Post)
    category = db.ReferenceProperty(Category)


class sss(webapp.RequestHandler):
  def get(self):
    obj1 = Post(title='hhaa')
    #obj1.title = 'haha'
    obj1.put()

    obj2 = Category()

    obj2.name='haha-kao'

    obj2.put()
    obj3=PostCategory()
    obj3.post=obj1
    obj3.category=obj2
    obj3.put()
    self.response.out.write(obj1.categories().get().name)

the error is :

Traceback (most recent call last):
  File "D:\Program Files\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 511, in __call__
    handler.get(*groups)
  File "D:\zjm_code\helloworld\a.py", line 131, in get
    self.response.out.write(obj1.categories().get().name)
AttributeError: 'generator' object has no attribute 'get'

so how to get the obj2.name via obj1's method

thanks