ForeignKey for one-to-many in Django

Introduction

A ForeignKey lets you link one item to many others. It helps organize related data clearly.

You want to connect many comments to one blog post.

You need to link multiple orders to one customer.

You want to assign many books to one author.

You want to track many tasks under one project.

Syntax

Django

class ModelName(models.Model):
    field_name = models.ForeignKey(OtherModel, on_delete=models.CASCADE)

OtherModel is the model you link to.

on_delete=models.CASCADE means if the linked item is deleted, related items are deleted too.

Examples

Each comment links to one post. If the post is deleted, its comments are deleted.

Django

class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    text = models.TextField()

Orders link to customers. If a customer is deleted, orders keep a null customer instead of deleting.

Django

class Order(models.Model):
    customer = models.ForeignKey(Customer, on_delete=models.SET_NULL, null=True)
    order_date = models.DateField()

Sample Program

This example shows an Author with many Books. Each Book links to one Author using ForeignKey.

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=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

# Example usage in Django shell:
# author = Author.objects.create(name='Jane Doe')
# book1 = Book.objects.create(title='Book One', author=author)
# book2 = Book.objects.create(title='Book Two', author=author)
# print(book1.author.name)
# print(book2.author.name)

Important Notes

Always set on_delete to decide what happens when the linked item is removed.

You can access all related items from the linked model using modelname_set by default (e.g., author.book_set.all()).

Summary

ForeignKey creates a one-to-many link between models.

It helps keep related data connected and organized.

Remember to choose on_delete behavior carefully.

Practice

1. In Django, where should you place a ForeignKey field to represent a one-to-many relationship?

easy

A. In the model representing the 'many' side of the relationship

B. In the model representing the 'one' side of the relationship

C. In both models to link them together

D. You don't use ForeignKey for one-to-many relationships

Solution

1. Step 1: Understand the one-to-many relationship

   One object on the 'one' side can relate to many objects on the 'many' side.

2. Step 2: Place ForeignKey on the 'many' side

   The ForeignKey field goes in the model that represents the 'many' side to link back to the 'one' side.

3. Final Answer:

   In the model representing the 'many' side of the relationship -> Option A

4. Quick Check:

   ForeignKey on 'many' side = A [OK]

Hint: ForeignKey always goes on the 'many' side model [OK]

Common Mistakes:

• Putting ForeignKey on the 'one' side model
• Adding ForeignKey to both models
• Thinking ForeignKey is not used for one-to-many

2. Which of the following is the correct syntax to define a ForeignKey in a Django model named Book that links to a model named Author?

easy

A. author = models.ForeignKey(Author, delete=models.CASCADE)

B. author = models.ForeignKey(Author, on_delete=models.CASCADE)

C. author = models.ForeignKey('Author')

D. author = models.ForeignKey(Author, on_delete='CASCADE')

Solution

1. Step 1: Check ForeignKey syntax

   ForeignKey requires the related model and the on_delete argument to specify delete behavior.

2. Step 2: Validate correct usage

   author = models.ForeignKey(Author, on_delete=models.CASCADE) correctly uses on_delete=models.CASCADE. Options A, B, and D have syntax errors or missing required arguments.

3. Final Answer:

   author = models.ForeignKey(Author, on_delete=models.CASCADE) -> Option B

4. Quick Check:

   Correct ForeignKey syntax = C [OK]

Hint: Always include on_delete=models.CASCADE with ForeignKey [OK]

Common Mistakes:

• Omitting on_delete argument
• Using wrong argument name like delete
• Passing on_delete as string instead of constant

3. Given these 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)

What will Book.objects.filter(author__name='Alice').count() return if there are 3 books by Alice and 2 by Bob?

medium

A. 0

B. 2

C. 5

D. 3

Solution

1. Step 1: Understand the filter query

   The query filters books where the related author's name is 'Alice'.

2. Step 2: Count matching books

   Since 3 books belong to Alice, the count will be 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Books by Alice = 3 [OK]

Hint: Filter on related model with double underscore __ [OK]

Common Mistakes:

• Counting authors instead of books
• Using single underscore instead of double
• Confusing author name with book title

4. What is wrong with this Django model code?

class Comment(models.Model):
    post = models.ForeignKey(Post)
    text = models.TextField()

medium

A. TextField cannot be used for text

B. ForeignKey should be named post_id

C. Missing on_delete argument in ForeignKey

D. ForeignKey should be in Post model

Solution

1. Step 1: Check ForeignKey arguments

   Since Django 2.0, on_delete is required for ForeignKey fields.

2. Step 2: Identify missing on_delete

   The code lacks on_delete, causing an error.

3. Final Answer:

   Missing on_delete argument in ForeignKey -> Option C

4. Quick Check:

   ForeignKey requires on_delete argument [OK]

Hint: Always add on_delete to ForeignKey fields [OK]

Common Mistakes:

• Forgetting on_delete causes errors
• Renaming ForeignKey field incorrectly
• Thinking TextField is invalid for text

5. You have these models:

class Category(models.Model):
    name = models.CharField(max_length=50)

class Product(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True)

If a Category is deleted, what happens to its related Products?

hard

A. Products remain but their category field is set to NULL

B. Products are deleted automatically

C. Deletion of Category is blocked if Products exist

D. Products keep the deleted category reference

Solution

1. Step 1: Understand on_delete=models.SET_NULL

   This option sets the ForeignKey field to NULL when the related object is deleted.

2. Step 2: Apply to Product-Category relation

   When a Category is deleted, related Products keep their records but their category field becomes NULL.

3. Final Answer:

   Products remain but their category field is set to NULL -> Option A

4. Quick Check:

   on_delete=SET_NULL means keep products, nullify category [OK]

Hint: SET_NULL keeps related objects, clears ForeignKey [OK]

Common Mistakes:

• Assuming related objects are deleted
• Thinking deletion is blocked
• Believing ForeignKey keeps deleted references