根据新近性从Django queryset中排除对象

Question

I have a reddit-like Django app where users can post interesting urls (links) and then publicly comment under them. The two data models to represent this are:

class Link(models.Model):
    description = models.TextField(validators=[MaxLengthValidator(500)])
    submitter = models.ForeignKey(User)
    submitted_on = models.DateTimeField(auto_now_add=True)

class Publicreply(models.Model):
    submitted_by = models.ForeignKey(User)
    answer_to = models.ForeignKey(Link)
    submitted_on = models.DateTimeField(auto_now_add=True)
    description = models.TextField(validators=[MaxLengthValidator(250)])

How do I query for all Links which have at least 1 or more publicreply , and secondly where the latest publicreply is not by self.request.user ? I sense something like the following:

Link.objects.filter(publicreply__isnull=False).exclude(**something here**)

Please advise! Performance is key too, hence the simpler the better!

Answer 1

For performance and simplicity you could cache both the number of replies and the latest reply:

class Link(models.Model):
    ...
    number_of_replies = models.PositiveIntegerField(default=0)
    latest_reply = models.ForeignKey('myapp.Publicreply', related_name='+', blank=True, null=True, on_delete=models.SET_NULL)

When a reply is entered, update the corresponding link.number_of_replies and link.latest_reply .

The query would then be:

Link.objects.filter(number_of_replies__gte=1)\
            .exclude(latest_reply__user=request.user)