ForeignKey for one-to-many in Django - Mini Project: Build & Apply

ForeignKey for one-to-many

📖 Scenario: You are building a simple blog application where each blog post can have many comments. You want to connect comments to their blog posts using Django's ForeignKey to show this one-to-many relationship.

🎯 Goal: Create two Django models: Post and Comment. Use a ForeignKey in Comment to link each comment to a single post. This will let you store multiple comments for each post.

📋 What You'll Learn

Create a Post model with a title field

Create a Comment model with a text field

Add a ForeignKey in Comment to Post with on_delete=models.CASCADE

Use related_name='comments' in the ForeignKey for easy access from Post

Follow Django model syntax and conventions

💡 Why This Matters

🌍 Real World

Blogs, forums, and social media apps often use one-to-many relationships to connect posts with comments or replies.

💼 Career

Understanding ForeignKey relationships is essential for backend developers working with Django to build relational data models.

1

Create the Post model

Create a Django model called Post with a single field title that is a CharField with max length 100.

Django

# Create the Post model with a title field
# Your code here

Hint

Use models.CharField(max_length=100) for the title field inside the Post model.

2

Create the Comment model

Create a Django model called Comment with a single field text that is a TextField.

Django

from django.db import models

class Post(models.Model):
    title = models.CharField(max_length=100)

# Create the Comment model with a text field
# Your code here

Hint

Use models.TextField() for the text field inside the Comment model.

3

Add ForeignKey to Comment

Add a ForeignKey field called post to the Comment model. It should link to the Post model, use on_delete=models.CASCADE, and have related_name='comments'.

Django

from django.db import models

class Post(models.Model):
    title = models.CharField(max_length=100)

class Comment(models.Model):
    text = models.TextField()
    # Add ForeignKey to Post here
    # Your code here

Hint

Use models.ForeignKey(Post, on_delete=models.CASCADE, related_name='comments') for the post field.

4

Add __str__ methods for readability

Add a __str__ method to both Post and Comment models. For Post, return the title. For Comment, return the first 20 characters of text.

Django

from django.db import models

class Post(models.Model):
    title = models.CharField(max_length=100)

    # Add __str__ method here
    # Your code here

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

    # Add __str__ method here
    # Your code here

Hint

Define __str__ methods to return self.title for Post and self.text[:20] for Comment.

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