How to annotate the Max of a related object to a Django queryset

Question

I have a Django website where users can make groups, and then write replies under the said groups.

Simply put:

class Group(models.Model):
    owner = models.ForeignKey(User)        
    #some attributes

class Reply(models.Model):
    text = models.TextField(validators=[MaxLengthValidator(500)])
    writer = models.ForeignKey(User)

For a given user, I want to find all groups that she either made, or ever replied under (even if once). Furthermore, from all such groups, I want to extract the ID of the most recent reply - whether made by the user herself, or by someone else.

To accomplish the above, I have tried:

groups = Group.objects.filter(Q(owner=self.request.user)|Q(reply__writer=self.request.user)).distinct()
ids = groups.annotate(max_reply=Max('reply__id')).values_list('max_reply', flat=True)

This doesn't work - in ids , I only get the most recent reply written by self.request.user herself, not the absolute most recent reply (written by anyone) under each group. I find that weird because seemingly, this ought to work, no? Maybe I'm referencing the reverse foreignkey incorrectly.

Can you help me fix this, or suggest an alternative?

Answer 1

The following is how I'm currently accomplishing it. I assign the ids of all relevant groups to groups . I find the ids of max replies under each and assign them to replies . Finally, I construct a queryset around 60 such replies, assigning that to replies_qs .

groups = Group.objects.filter(Q(owner=self.request.user)|Q(reply__writer=self.request.user)).distinct().values_list('id', flat=True)
replies = Reply.objects.filter(which_group__in=groups).values('which_group_id').annotate(Max('id')).values_list('id__max', flat=True)
replies_qs = Reply.objects.filter(id__in=replies)[:60]

This is 3 DB accesses, would prefer to do it in less, if someone has a suggestion.