def post_image_path(instance, filename):
     id = instance.post_id
     if id == None:
         try:
             id = max(map(lambda a: a.post_id, Post.objects.all())) + 1
         except ValueError as error:
             id = 1
     return os.path.join('threads/', str(id), filename)

def post_image_path(instance, filename):
       id = instance.post_id
       if id == None:
           id = Post.objects.all().aggregate(Max('post_id'))
           if id['post_id__max'] == None:
               id['post_id__max'] = 1
           else:
               id['post_id__max'] = id['post_id__max'] + 1
       return os.path.join('threads/', str(id['post_id__max']), filename)