Relationship query patterns in Django

Introduction

Relationship query patterns help you get data connected between tables easily. They let you find related information without writing complex code.

You want to get all books written by a specific author.

You need to find all comments related to a blog post.

You want to list all orders made by a customer.

You want to filter users who have posted at least one article.

You want to access the profile details linked to a user.

Syntax

Django

Model.objects.filter(related_model__field=value)
Model.objects.get(pk=1).related_model_set.all()

Use double underscores (__) to follow relationships in queries.

Related objects can be accessed using the related model name plus '_set' by default.

Examples

Find all books where the author's name is 'Alice'.

Django

Book.objects.filter(author__name='Alice')

Get all books written by the author with primary key 1.

Django

author = Author.objects.get(pk=1)
author.book_set.all()

Find comments on posts with 'django' in the title (case-insensitive).

Django

Comment.objects.filter(post__title__icontains='django')

Get users whose linked profile has age 18 or older.

Django

User.objects.filter(profile__age__gte=18)

Sample Program

This example shows how to create an author and two books linked to her. Then it queries all books where the author's name is 'Alice' and prints their titles.

Django

from django.db import models

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)

# Sample usage
# Create an author and books
alice = Author.objects.create(name='Alice')
Book.objects.create(title='Django Basics', author=alice)
Book.objects.create(title='Advanced Django', author=alice)

# Query books by author name
books_by_alice = Book.objects.filter(author__name='Alice')

for book in books_by_alice:
    print(book.title)

Important Notes

Remember to use the related model's field names exactly as defined.

Using double underscores lets you go through multiple relationships, like book__author__name.

Accessing related objects backward uses relatedmodel_set unless you set related_name.

Summary

Relationship queries let you find connected data easily.

Use double underscores to follow links between models.

Access related objects forward and backward with simple patterns.