Primary key behavior in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the primary key value after saving a new Django model instance?

Consider this Django model and code snippet:

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

book = Book(title='Django Basics')
book.save()
print(book.pk)

What will print(book.pk) output?

Django

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

book = Book(title='Django Basics')
book.save()
print(book.pk)

A The integer ID assigned by the database after saving, e.g., 1

B None, because the primary key is not set until manually assigned

C Raises an AttributeError because pk is not defined

D The title string 'Django Basics' as the primary key

💡 Hint

Think about what Django does when you save a new model instance without specifying a primary key.

Django automatically creates an integer primary key field called id if none is specified. When you save a new instance, the database assigns this ID, which is accessible via pk.

📝 Syntax

intermediate

Which model definition correctly sets a custom primary key in Django?

Choose the correct Django model code that defines a username field as the primary key.

A

class User(models.Model):
    username = models.CharField(max_length=30, primary_key=True)
    email = models.EmailField()

B

class User(models.Model):
    username = models.CharField(max_length=30)
    email = models.EmailField()
    primary_key = 'username'

C

class User(models.Model):
    username = models.CharField(max_length=30)
    email = models.EmailField()

    class Meta:
        primary_key = 'username'

D

class User(models.Model):
    username = models.CharField(max_length=30, unique=True)
    email = models.EmailField()

💡 Hint

Remember how to mark a field as primary key in Django model fields.

To set a custom primary key, you add primary_key=True to the field definition. Options B and C are invalid syntax, and D only makes the field unique but not a primary key.

🔧 Debug

advanced

Does this Django model raise an error on migration?

Given this model:

class Product(models.Model):
    sku = models.CharField(max_length=20, primary_key=True)
    name = models.CharField(max_length=100)

class Order(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField()

Running migrations: does it raise an error about missing primary key?

Django

class Product(models.Model):
    sku = models.CharField(max_length=20, primary_key=True)
    name = models.CharField(max_length=100)

class Order(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField()

A ForeignKey must reference a field named 'id' by default, so referencing 'sku' causes error

B The Product model must have an explicit 'id' field even if 'sku' is primary key

C No error occurs; the code is valid and migrations run fine

D The ForeignKey needs to specify 'to_field="sku"' to link to the custom primary key

💡 Hint

Think about how ForeignKey determines which field to reference on the target model when it has a custom primary key.

No error occurs; the code is valid and migrations run fine. Django's ForeignKey automatically references the primary key field of the target model ('sku'), regardless of name. to_field is only required when referencing a non-primary-key field.

❓ state_output

advanced

What is the value of the primary key after updating a Django model instance?

Given this code:

class Customer(models.Model):
    email = models.EmailField(primary_key=True)
    name = models.CharField(max_length=50)

c = Customer(email='[e-mail]', name='Alice')
c.save()
c.name = 'Alice Smith'
c.save()
print(c.pk)

What will print(c.pk) output?

Django

class Customer(models.Model):
    email = models.EmailField(primary_key=True)
    name = models.CharField(max_length=50)

c = Customer(email='[e-mail]', name='Alice')
c.save()
c.name = 'Alice Smith'
c.save()
print(c.pk)

A None, because updating clears the primary key

B '[e-mail]', the primary key remains the same after update

C Raises a ValueError because primary key cannot be updated

D The name 'Alice Smith' because primary key changes to last saved field

💡 Hint

Think about whether the primary key changes when you update other fields.

The primary key value remains the same unless explicitly changed. Updating other fields and saving does not affect the primary key.

🧠 Conceptual

expert

What happens if you define two fields with primary_key=True in a Django model?

Consider this model:

class Employee(models.Model):
    emp_id = models.IntegerField(primary_key=True)
    badge_id = models.CharField(max_length=10, primary_key=True)
    name = models.CharField(max_length=50)

What will happen when you try to run migrations?

Django

class Employee(models.Model):
    emp_id = models.IntegerField(primary_key=True)
    badge_id = models.CharField(max_length=10, primary_key=True)
    name = models.CharField(max_length=50)

A Migrations succeed but database throws an error at runtime

B Creates a composite primary key with both fields combined

C Only the first primary_key=True field is used; the second is ignored

D Raises a SystemCheckError because Django models cannot have multiple primary keys

💡 Hint

Recall Django's rules about primary keys in models.

Django does not support multiple primary keys on a model. Defining more than one field with primary_key=True causes a system check error during migration creation.