Revision history [back]

Prefetching related objects in Django allows you to retrieve related objects for a queryset or a list of model instances at once, instead of making one query for each related object. This can improve performance, especially when dealing with large datasets.

To use prefetch with related_name relation in Group by, follow these steps:

1. Define your models and related objects:

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=100)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books')

1. Retrieve a queryset of authors along with their books using prefetch_related:

authors = Author.objects.prefetch_related('books').all()

1. Group the authors by name using annotate:

from django.db.models import Count

authors_by_name = Author.objects.annotate(num_books=Count('books')).order_by('-num_books')

1. Use the values() method to select only the fields you need:

authors_by_name = authors_by_name.values('id', 'name', 'num_books')

1. Serialize the queryset or use it in your view as needed. Here's an example using Django REST framework:

from rest_framework import serializers

class BookSerializer(serializers.ModelSerializer):
    class Meta:
        model = Book
        fields = ('id', 'title')

class AuthorSerializer(serializers.ModelSerializer):
    num_books = serializers.IntegerField()
    books = BookSerializer(many=True)

    class Meta:
        model = Author
        fields = ('id', 'name', 'num_books', 'books')

serializer = AuthorSerializer(authors_by_name, many=True)

This will give you a serialized queryset of authors sorted by the number of books they have, along with a list of their books. The related books will be pre-fetched using the 'books' related_name.