Model relationships preview in Django

Introduction

Model relationships let you connect data tables in Django. This helps organize and link related information easily.

You want to link a blog post to its author.

You need to connect products to categories in a store.

You want to track which users liked which comments.

You want to store orders linked to customers.

You want to create a many-to-many connection like students and courses.

Syntax

Django

class ModelName(models.Model):
    foreign_key_field = models.ForeignKey(OtherModel, on_delete=models.CASCADE)
    one_to_one_field = models.OneToOneField(OtherModel, on_delete=models.CASCADE)
    many_to_many_field = models.ManyToManyField(OtherModel)

ForeignKey creates a many-to-one link.

OneToOneField creates a one-to-one link.

ManyToManyField creates a many-to-many link.

Examples

Each book links to one author. Many books can have the same author.

Django

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

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=200)

Each user has exactly one profile linked one-to-one.

Django

class UserProfile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    bio = models.TextField()

Students and courses have a many-to-many relationship.

Django

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

class Course(models.Model):
    students = models.ManyToManyField(Student)
    title = models.CharField(max_length=200)

Sample Program

This example shows a book linked to an author using ForeignKey. When you print book.author.name, it shows the author's name.

Django

from django.db import models

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

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=200)

# Usage example (in Django shell):
# author = Author.objects.create(name='Alice')
# book = Book.objects.create(author=author, title='Django Basics')
# print(book.author.name)

Important Notes

Always set on_delete to decide what happens if the linked object is deleted.

Use related_name to customize reverse lookups.

Remember to run migrations after changing models.

Summary

Model relationships connect data tables in Django.

ForeignKey is many-to-one, OneToOneField is one-to-one, ManyToManyField is many-to-many.

They help organize and access related data easily.