On_delete options (CASCADE, PROTECT, SET_NULL) in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a parent object is deleted with CASCADE?

Consider a Django model where a child model has a ForeignKey to a parent model with on_delete=models.CASCADE. What is the behavior when the parent object is deleted?

Django

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

class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)

A All child objects linked to the parent are deleted automatically.

B The deletion of the parent is blocked and raises an error.

C The parent is deleted but child objects keep their foreign key pointing to a non-existent parent.

D The foreign key in child objects is set to NULL, but the child objects remain.

💡 Hint

Think about what CASCADE means in everyday life, like a waterfall effect.

CASCADE means when the parent is deleted, all related child objects are also deleted automatically. This prevents orphaned child records.

❓ component_behavior

intermediate

What error occurs with PROTECT on deleting a parent?

If a ForeignKey uses on_delete=models.PROTECT, what happens when you try to delete the parent object that has child objects linked to it?

Django

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

class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.PROTECT)
    name = models.CharField(max_length=100)

A The parent is deleted and child objects are also deleted.

B The parent is deleted and child foreign keys are set to NULL.

C A ProtectedError is raised and the parent deletion is blocked.

D The parent is deleted but child foreign keys point to a missing parent.

💡 Hint

PROTECT means to stop something from happening.

PROTECT prevents deletion of the parent if any child objects exist. Django raises a ProtectedError to block the deletion.

❓ component_behavior

advanced

What is the effect of SET_NULL on deleting a parent object?

Given a ForeignKey with on_delete=models.SET_NULL and null=True, what happens to child objects when the parent is deleted?

Django

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

class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.SET_NULL, null=True)
    name = models.CharField(max_length=100)

A Child objects are deleted along with the parent.

B Child objects keep their foreign key pointing to the deleted parent.

C Deletion of the parent is blocked with an error.

D Child objects remain and their foreign key is set to NULL.

💡 Hint

Think about removing a link but keeping the child safe.

SET_NULL sets the foreign key in child objects to NULL when the parent is deleted, so children remain but without a parent link.

📝 Syntax

advanced

Which ForeignKey declaration is correct for SET_NULL?

You want to use on_delete=models.SET_NULL in a ForeignKey. Which option is syntactically correct and will work without errors?

A parent = models.ForeignKey(Parent, on_delete=models.SET_NULL, blank=True)

B parent = models.ForeignKey(Parent, on_delete=models.SET_NULL, null=True)

C parent = models.ForeignKey(Parent, on_delete=models.SET_NULL)

D parent = models.ForeignKey(Parent, on_delete=models.SET_NULL, default=None)

💡 Hint

SET_NULL requires the field to accept NULL values.

Using SET_NULL requires null=True so the database allows NULL values. Without it, a database error occurs.

❓ state_output

expert

What is the count of Child objects after deleting a Parent with mixed on_delete behaviors?

Given these models and data, what is the number of Child objects remaining after deleting the Parent with id=1?

Django

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

class ChildCascade(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)

class ChildProtect(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.PROTECT)

class ChildSetNull(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.SET_NULL, null=True)

# Setup:
# Parent object with id=1 exists
# ChildCascade objects linked to Parent 1: 3
# ChildProtect objects linked to Parent 1: 2
# ChildSetNull objects linked to Parent 1: 4

# Action:
# Attempt to delete Parent with id=1

A Deletion fails with ProtectedError; all 9 child objects remain

B 4 child objects remain: only SetNull children with foreign keys set to NULL

C 5 child objects remain: 2 Protect and 3 SetNull (SetNull children foreign keys set to NULL)

D 9 child objects remain: 3 Cascade, 2 Protect, 4 SetNull

💡 Hint

PROTECT blocks deletion if any child exists.

Because ChildProtect uses PROTECT and there are 2 such children, deleting the parent raises ProtectedError and no deletion happens. All children remain.