Model relationships preview in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of accessing related objects in Django?

Given these Django 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, related_name='books')

What will author.books.count() return after creating one author and two books linked to that author?

Django

author = Author.objects.create(name='Alice')
Book.objects.create(title='Book 1', author=author)
Book.objects.create(title='Book 2', author=author)
result = author.books.count()

A 1

B 2

C 0

D Raises AttributeError

💡 Hint

Think about how the related_name works for reverse lookups.

The related_name='books' on the ForeignKey allows accessing all Book objects linked to an Author via author.books. Since two books are created for the author, author.books.count() returns 2.

📝 Syntax

intermediate

Which option correctly defines a ManyToMany relationship in Django?

You want to model a relationship where multiple students can enroll in multiple courses. Which model definition is correct?

A

class Student(models.Model):
    courses = models.OneToOneField('Course', on_delete=models.CASCADE)

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

B

class Student(models.Model):
    courses = models.ForeignKey('Course', on_delete=models.CASCADE)

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

C

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

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

D

class Student(models.Model):
    courses = models.ManyToManyField('Course')

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

💡 Hint

Many students can have many courses, so think about the field type that supports many-to-many.

ManyToManyField is the correct field to represent many-to-many relationships in Django. ForeignKey is one-to-many, OneToOneField is one-to-one, and CharField is just text.

🔧 Debug

advanced

Why does accessing a related object raise DoesNotExist error?

Given these models:

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

And this code:

profile = Profile.objects.get(user=request.user)
print(profile.bio)

Sometimes this raises Profile.DoesNotExist. Why?

A The user does not have a linked Profile object in the database.

B The OneToOneField is incorrectly defined and causes a syntax error.

C The bio field is empty, causing the error.

D The User model does not exist in Django by default.

💡 Hint

Think about what happens if the related object is missing.

The DoesNotExist error means the query found no Profile linked to the user. This happens if the user has no Profile record yet.

❓ state_output

advanced

What is the output of this code using select_related?

Given these models:

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

class Book(models.Model):
    title = models.CharField(max_length=100)
    publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)

And this code:

book = Book.objects.select_related('publisher').get(id=1)
print(book.publisher.name)

What does select_related do here?

A It raises a ValueError because select_related cannot be used with ForeignKey.

B It delays fetching the Publisher until publisher.name is accessed, causing a second query.

C It fetches the Book and its Publisher in one database query, so accessing publisher.name does not cause another query.

D It fetches only the Book and ignores the Publisher.

💡 Hint

select_related is used to optimize queries by joining related tables.

select_related performs a SQL join to get related objects in one query. This avoids extra queries when accessing related fields.

🧠 Conceptual

expert

How does Django handle cascading deletes in model relationships?

Consider these models:

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

class Item(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)

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

A All Items linked to the deleted Category are also deleted automatically.

B The Items remain but their category field is set to NULL.

C Deleting Category raises an IntegrityError because Items depend on it.

D The Items are archived but not deleted.

💡 Hint

Check the meaning of on_delete=models.CASCADE.

on_delete=models.CASCADE means deleting the referenced object (Category) will delete all related objects (Items) automatically.