Django Annotate and Aggregate - django-models

Question

I'd like to Sum the post_value of all of the Posts for each post_user to eventually use in a chart. I'm struggling with how to formulate the query?
So far, I've got to:
user_totals = User.objects.annotate(post_value_total=Sum('post'))
models.py
class User(AbstractUser):
pass
class Post(models.Model):
post_user = models.ForeignKey(User, on_delete=models.CASCADE)
post_cat = models.ForeignKey(Category, on_delete=models.CASCADE)
post_action = models.ForeignKey(Action, on_delete=models.CASCADE)
post_quantity = models.PositiveIntegerField(blank=True, null=True)
post_value = models.PositiveIntegerField(default='0')
post_timestamp = models.DateTimeField(auto_now_add=True)
def __str__(self):
return f"{self.post_user}'s post at {self.post_timestamp}"
Thanks.

Answer 1

I'd like to Sum the post_value of all of the Posts for each post_user to eventually use in a chart.
Since each Post has a non-nullable post_user ForeignKey, it means that each Post belongs to exactly one user.
We thus can sum up the number of post_values of all Users with:
Post.objects.all().count()
If you only want to sum these up for a subset of the users, you can work with:
Post.objects.filter(
post_user__in=[user1, user2, user3]
).count()
or if you have ids:
Post.objects.filter(
post_user_id__in=[user_id1, user_id2, user_id3]
).count()
Or if you want to sum up the post_values, you can work with:
from django.db.models import Sum
total_post_value = Post.objects.aggregate(
total=Sum('post_value')
) or 0
The or 0 is necessary if the collection can be empty, since the sum of no records is NULL/None, not 0.
Or if you want to do this per User, we can work with:
user_totals = User.objects.annotate(
post_value_total=Sum('post__post_value')
)
The User objects that arise from this will have an extra attribute post_value_total that sums up the values of the related Posts. These can be None if a user has no related Posts. In that case we can work Coalesce [Django-doc]:
from django.db.models import Sum, Value
from django.db.models.functions import Coalesce
user_totals = User.objects.annotate(
post_value_total=Coalesce(Sum('post__post_value'), Value(0))
)