ForeignKey for one-to-many in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Django model relationship produce?

Given these Django models, what will be the output of print(book.author.name) if book is an instance of Book linked to an Author named 'Alice'?

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)

# Assume book is a Book instance linked to Author named 'Alice'
print(book.author.name)

A Raises AttributeError because 'author' is not accessible

B "Alice"

C "book.author"

D None

💡 Hint

Remember that ForeignKey creates a link to a single related object.

The ForeignKey field 'author' in Book links to one Author instance. Accessing 'book.author.name' returns the name of that Author, which is 'Alice'.

❓ state_output

intermediate

How many books are related to this author?

Given these models and an Author instance author with 3 related Book instances, what is the output of print(author.book_set.count())?

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)

# Assume author is an Author instance with 3 books linked
print(author.book_set.count())

A Raises AttributeError because 'book_set' does not exist

B 0

C 3

D Raises TypeError

💡 Hint

Django automatically creates a reverse relation named modelname_set.

The reverse relation from Author to Book is accessible via 'book_set'. Counting it returns the number of related Book instances, which is 3.

📝 Syntax

advanced

Which option correctly defines a ForeignKey for one-to-many in Django?

Select the correct Django model field definition to create a one-to-many relationship from Book to Author.

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)
    # ForeignKey field here

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

B author = models.CharField(max_length=100)

C author = models.ManyToManyField(Author)

D author = models.OneToOneField(Author, on_delete=models.CASCADE)

💡 Hint

One-to-many uses ForeignKey, not OneToOneField or ManyToManyField.

ForeignKey creates a many-to-one relationship, which is one-to-many from Author to Book. OneToOneField is one-to-one, ManyToManyField is many-to-many, and CharField is just text.

🔧 Debug

advanced

Why does this code raise an error?

Given these models, why does accessing book.author_set.all() raise an error?

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)

book = Book.objects.first()
print(book.author_set.all())

A 'Author' object has no attribute 'author_set' because of typo

B None, code runs without error

C SyntaxError due to missing parentheses in ForeignKey

D 'Book' object has no attribute 'author_set' because reverse relation is on Author, not Book

💡 Hint

Reverse relations are accessed from the model on the 'one' side, not the 'many' side.

The reverse relation 'author_set' is created on Author instances to access related Books, not on Book instances. 'book.author_set' does not exist.

🧠 Conceptual

expert

What happens when you delete an Author instance linked to Books with on_delete=models.CASCADE?

Consider these models with author = models.ForeignKey(Author, on_delete=models.CASCADE). What is the effect on related Book instances when an Author is deleted?

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)

A All Book instances linked to the deleted Author are also deleted

B Book instances remain but their author field is set to NULL

C Deletion of Author is blocked if any Book is linked

D Book instances remain unchanged with author field still pointing to deleted Author

💡 Hint

on_delete=models.CASCADE means related objects are deleted too.

CASCADE deletes all related objects automatically when the referenced object is deleted, so all Books linked to that Author are deleted.