如何使一个模型中的函数像一个字段一样工作？

Question

我有一个Post模型，通过ForeignKey的链接问心无愧PostScore：

class Post(models.Model): 
user = models.ForeignKey(User, blank=True, null=True) 
title = models.TextField(max_length=76) 
... 


class PostScore(models.Model): 
    user = models.ForeignKey(User, blank=True, null=True) 
    post = models.ForeignKey(Post, related_name='score') 
    upvotes = models.IntegerField(default=0) 
    downvotes = models.IntegerField(default=0) 

    def trending(self): 
     score = self.upvotes - self.downvotes 
     return score 

所以，当我尝试我的帖子是这样排序：

posts = Post.objects.all().order_by('-score__upvotes') 

它正常工作，但如何将能够排序通过trending：

posts = Post.objects.all().order_by('-score__trending') 

上面的代码创建了这个错误：

FieldError at /news/ 
Cannot resolve keyword 'trending' into field. Choices are: downvotes, id, post, post_id, upvotes, user, user_id 

Answer 1

Calculate the score difference with annotate method and then order by with that calculated field.

Post.objects.annotate(score_diff=F('upvotes') - F('downvotes')).order_by('-score_diff')