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 Table
id name 1 Charlie 2 Alice 3 Zoe
Book Table
id title author_id 1 Book A 1 2 Book B 2 3 Book C 2 4 Novel D 3
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