Avoiding duplicate objects in Django querysets

Summary

When filtering Django querysets across relationships, you can easily end up with duplicate objects in your results. This is a common gotcha that happens with both one-to-many (1:N) and many-to-many (N:N) relationships. Let’s explore why this happens and the best way to avoid it.The Problem#When you filter a queryset by traversing a relationship, Django performs a SQL JOIN. If a parent object has multiple related objects that match your filter, the parent object appears multiple times in the result set.Let’s look at a concrete example with a one-to-many relationship:class Author(models.Model): name = models.CharField(max_length=255) class Book(models.Model): title = models.CharField(max_length=255) author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books') Here’s what the database tables might look like:Author TableBook Tableidtitleauthor_id1Book A12Book B23Book C24Novel D3If we want to find all authors who have written books whose titles start with “Book”:Author.objects.filter(books__title__startswith="Book") # [<Author: Charlie>, <Author: Alice>, <Author: Alice>] Notice that Alice appears twice in the result set. This happens because when Django executes the query, it performs a JOIN across the relationship:author_idnamebook_idtitle1Charlie1Book A2Alice2Book B2Alice3Book CSince Alice wrote both “Book B” and “Book C”, the JOIN produces two rows for her. The same issue occurs with many-to-many relationships — if an object belongs to multiple groups that match your filter, the object will appear multiple times.Common Solutions (and Their Problems)#Using distinct()#The most straightforward solution is to use distinct():Author.objects.filter(books__title__startswith="Book").distinct() # [<Author: Charlie>, <Author: Alice>] This works, but can be expensive. distinct() compares all selected fields to determine uniqueness, which is problematic if your model has large fields like JSONField or TextField. The database has to compare all these fields fo...