[英]Django order_by ForeignKey set models
I have following django models 我有以下的Django模型
class Post(models.Model):
title = models.CharField(max_length=240)
class Comment(models.Model):
post = models.ForeignKey(Post)
date = models.DateTimeField(auto_now_add=True)
I need a QuerySet of comments, ordered first by post, then by date. 我需要一个QuerySet注释,首先按帖子排序,然后按日期排序。 But posts must be ordered by its latest comment.
但是帖子必须按其最新评论排序。
If i could use model methods in QuerySet order_by, it would be like this: 如果我可以在QuerySet order_by中使用模型方法,它将像这样:
class Post(models.Model):
title = models.CharField(max_length=240)
def get_last_comment_date(self):
return self.comment_set.order_by('-date')[0].date
And the ordering, that i needed, could be: 我需要的排序可能是:
Comment.objects.all().order_by('post__get_last_comment_date', '-date')
But unfortunately, methods in order_by are not allowed. 但不幸的是,不允许使用order_by中的方法。
Please, help. 请帮忙。 Can i have such ordering?
我可以订购吗?
You may not use methods in order_by lookups because they are converted to SQL . 您不能在order_by 查找中使用方法,因为它们已转换为SQL 。
So, why not convert get_last_comment_date into a field ? 那么,为什么不将get_last_comment_date转换为字段呢? eg using a signal receiver :
例如使用信号接收器 :
from django.db.models import signals
class Post(models.Model):
title = models.CharField(max_length=240)
last_comment_date = models.DateField(null=True, blank=True)
def post_last_comment_date(sender, instance=None, **kwargs):
try:
last_comment_date = self.comment_set.order_by('-date')[0].date
except Comment.DoesNotExist:
return
if last_comment_date != comment.post.last_comment_date:
comment.post.last_comment_date = last_comment_date
comment.post.save()
signals.post_save.connect(post_last_comment_date, sender=Comment)
Now, you can: Comment.objects.order_by('post__last_comment_date', '-date')
现在,您可以:
Comment.objects.order_by('post__last_comment_date', '-date')
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