Django QuerySet Prefetch Related optimization guide as Expected with Complex Filters
I'm performance testing and I'm experimenting with I'm upgrading from an older version and I'm working on a Django application (version 3.2) where I need to optimize database queries using `prefetch_related`..... However, I'm working with unexpected behavior when trying to filter the related objects. I'm trying to fetch a list of `Author` objects along with their related `Book` objects, but only those books that were published after 2020. Here's a simplified version of my models: ```python 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) published_date = models.DateField() ``` I attempted to use `prefetch_related` like this: ```python from django.db.models import Prefetch from datetime import date authors = Author.objects.prefetch_related( Prefetch('book_set', queryset=Book.objects.filter(published_date__gt=date(2020, 1, 1))) ) ``` However, when I iterate through the authors and their books, I'm finding that some authors still have books from before 2020 in their related set. This is unexpected, as I assumed the filtering would apply correctly. ```python for author in authors: print(f'Author: {author.name}') for book in author.book_set.all(): # This still shows pre-2020 books! print(f' Book: {book.title}, Published: {book.published_date}') ``` I’ve also tried accessing the prefetch directly using `author.book_set.all()` after the `prefetch_related`, but the same scenario occurs. I've double-checked that the `published_date` field is populated correctly and no books should be eligible for display if the filter works. Can anyone clarify why the filtering isn't behaving as expected in this scenario? Is there an scenario with how `prefetch_related` handles the filtering, or is there something I'm missing in the queryset logic? Thanks in advance! I'm using Python latest in this project. Am I missing something obvious?