Annotating single model instance (instead of queryset) in django

Question

So, I have a Post model which contains PostVotes from users

class Post(models.Model):
    voters = models.ManyToManyField(User, through="PostVote")
    #other stuff

and the post vote can have a state of either "upvote" or "downvote" (I know I should be using enums or a bool for this before I start receiving those comments) and in many cases I need to count the total score of the object for the frontend. When I have the posts in a queryset, the following solution is working well:

posts = Post.objects.all().annotate(vote=models.Sum(
    models.Case(
        models.When(postvote__state="upvote", then=1),
        models.When(postvote__state="downvote", then=-1),
        default=0,
        output_field=models.IntegerField()
    )
))

However, there are many cases where I want to do a similar thing but instead of a queryset I have just a single instance. How do I do this? Trying the above solution gives 'Post' object has no attribute 'annotate'

jkoestinger · Accepted Answer · 2021-07-23 13:11:49Z

2

If I understood you already have a Post instance, and you would like to get its score? I so, I think you could implement a computed property as follow:

class Post(models.Model):
    voters = models.ManyToManyField(User, through="PostVote")

    @property
    def score(self):
        return sum([-1 if vote.state == 'downvote' else 1 for vote in self.postvotes.all()])

Note that this method will trigger extra DB queries if you didn't fetch your Post instance with .prefetch_related('postvotes')

Then you can use post.score to get the result

Another way to implement the property would be by doing a query instead:

class Post(models.Model):
    voters = models.ManyToManyField(User, through="PostVote")

    @property
    def score(self):
        return PostVote.objects.filter(post=self.pk).aggregate(vote=models.Sum(
            models.Case(
                models.When(state="upvote", then=1),
                models.When(state="downvote", then=-1),
                default=0,
                output_field=models.IntegerField()
            )
        ))['vote']

Note that I didn't try any of those code, so there might be any typo/wrong variable names

edited Jul 23, 2021 at 13:11

answered Jul 23, 2021 at 12:52

jkoestinger

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3 Comments

Piturnah Over a year ago

Thanks! This looks like the right solution. However, when trying the computed property I'm getting the error 'Post' object has no attribute 'postvotes'. So far I've tried also using postvote, voters, and tried adding a related_name= and using that, none of which have worked. Any idea where this is going wrong?

jkoestinger Over a year ago

on the PostVote class, you should have something like post = models.ForeignKey(Post, related_name='postvotes') and use self.postvotes.all() (I think I forgot the .all() part, I'll add it through edition)

Ivan Rostovsky Over a year ago

But what is about code duplication? Ie for now you have the score method implemented in 2 places: in queryset(or even manager) and in instance. In case of logic changing it will be a trick to do a change in 2 places. PS: I also face with this problem, no solution except duplication and just adding the comment in code about 2-nd place.

S.D. · Accepted Answer · 2023-06-24 05:28:36Z

Another solution might be to add it to your query sets by default. This should make the annotation available throughout.

Create a file managers.py inside of your app. And create the custom queryset/manager

from django.db import models

class PostQuerySet(models.QuerySet):
    def annotate_vote(self):
      return self.annotate(vote=models.Sum(
          models.Case(
            models.When(postvote__state="upvote", then=1),
            models.When(postvote__state="downvote", then=-1),
            default=0,
            output_field=models.IntegerField()
        )))

class PostManager(models.Manager):
    def get_queryset(self):
        return PostQuerySet(self.model, using=self._db).\
            annotate_vote()

Add the new manager as inside of your model.

from .managers import PostManager

class Post(models.Model):
    ...
    objects = PostManager()
    ...

This should give you accesss to the data like so:

Post.objects.last().vote

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