How can I use annotate in Django models?

How can I use annotate in Django models?

You can use the annotate() function in Django to add aggregate values to your querysets. It allows you to perform calculations on your data.

Can you give me an example of how to use it?

Sure! If you have a model called Book and you want to count the number of reviews for each book, you can do something like this:

from django.db.models import Count

books = Book.objects.annotate(num_reviews=Count('reviews'))

This will add a num_reviews attribute to each Book instance.

What if I want to filter the results after annotating?

You can chain the filter() method after annotate(). For example:

books_with_reviews = books.filter(num_reviews__gt=0)

This will give you only the books that have at least one review.