How can I use aggregate with annotations in Django?

How can I use aggregate with annotations in Django?

You can use the annotate() method to add calculated fields to your queryset, and then use aggregate() to perform calculations on those fields.

Can you give me an example of how to do that?

Sure! For instance, if you have a Book model and you want to count the number of reviews per author, you can do:

from django.db.models import Count

Book.objects.values('author').annotate(review_count=Count('reviews'))

What if I want to calculate the average rating of those reviews?

You can chain annotate() with aggregate(). For example:

from django.db.models import Avg

Book.objects.values('author').annotate(avg_rating=Avg('reviews__rating'))

Thanks! Is there a way to filter the results?

Yes, you can use the filter() method before annotate(). For example:

Book.objects.filter(published=True).values('author').annotate(avg_rating=Avg('reviews__rating'))