Django Order_by Foreignkey Set Models

I have following django models class Post(models.Model): title = models.CharField(max_length=240) class Comment(models.Model): post = models.ForeignKey(Post) date = mo

Solution 1:

You may not use methods in order_by lookups because they are converted to SQL.

So, why not convert get_last_comment_date into a field ? e.g. using a signal receiver:

from django.db.models import signals

classPost(models.Model):
    title = models.CharField(max_length=240)
    last_comment_date = models.DateField(null=True, blank=True)

defpost_last_comment_date(sender, instance=None, **kwargs):
    try:
        last_comment_date = self.comment_set.order_by('-date')[0].date
    except Comment.DoesNotExist:
        returnif 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')