无法设置ImageField url属性

class Society(models.Model):
     name = models.CharField(max_length=200)
     slug = models.SlugField(unique=True)
     summary = models.TextField(blank=True,null=True)
     members = models.ManyToManyField(User,null=True,blank=True)
     gallery = models.ForeignKey(Gallery,null=True,blank=True)
     avatar = models.ImageField(upload_to=get_society_path)

     def save(self,*args,**kwargs):
          super(Society, self).save(*args,**kwargs)
          fix_avatar_path(self)

     def clean(self):
          if self.id:
               self.avatar.path = get_society_path(self,self.avatar.path)
               save_thumb(self.avatar.path)

def get_society_path(instance,filename):
     seperator_val = instance.id
     if seperator_val is None:
          seperator_val = get_time()
     return '%s/society_%s/%s' % (settings.UPLOAD_ROOT,seperator_val,time_to_name(filename))

def fix_avatar_path(instance):
     org_society_path = get_society_path(instance,instance.avatar.name)
     make_upload_dir(org_society_path)
     move(instance.avatar.path,org_society_path)
     os.rmdir(os.path.dirname(instance.avatar.path))
     instance.clean()