Self-referencing relationships in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Django model's __str__ method?

Consider this Django model with a self-referencing ForeignKey. What will be printed when calling print(category) if category.name = 'Books' and category.parent = None?

Django

from django.db import models

class Category(models.Model):
    name = models.CharField(max_length=100)
    parent = models.ForeignKey('self', null=True, blank=True, on_delete=models.CASCADE)

    def __str__(self):
        return f"{self.name} - Parent: {self.parent.name if self.parent else 'None'}"

A Books - Parent: None

B Books - Parent: Books

C Books - Parent: ''

D Books - Parent: Null

💡 Hint

Check how the __str__ method handles the parent attribute when it is None.

The __str__ method returns the category name and the parent's name if it exists. Since parent is None, it prints 'None'.

❓ state_output

intermediate

What is the value of child.parent.name after saving these objects?

Given this Django model and code, what is the value of child.parent.name after execution?

Django

from django.db import models

class Node(models.Model):
    name = models.CharField(max_length=50)
    parent = models.ForeignKey('self', null=True, blank=True, on_delete=models.CASCADE)

root = Node.objects.create(name='Root')
child = Node.objects.create(name='Child', parent=root)

result = child.parent.name

A Root

B Child

C None

D Raises AttributeError

💡 Hint

Look at how the parent is assigned when creating the child node.

The child node's parent is set to the root node, so accessing child.parent.name returns 'Root'.

📝 Syntax

advanced

Which option correctly defines a self-referencing ManyToManyField in Django?

You want to create a Django model where each person can have many friends, and friends are also Person instances. Which code snippet correctly defines this self-referencing relationship?

Django

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=100)
    friends = ???

A models.ManyToManyField('self', symmetrical=False, blank=True)

B models.ManyToManyField('Person', symmetrical=False)

C models.ManyToManyField('self', symmetrical=True, blank=True)

D models.ForeignKey('self', on_delete=models.CASCADE)

💡 Hint

Friends usually have a symmetrical relationship.

For friends, the relationship is symmetrical, so symmetrical=True is correct. Using 'self' as the model name is required for self-referencing.

🔧 Debug

advanced

Why does this self-referencing model raise a ValueError?

This Django model raises a ValueError when running migrations. What is the cause?

Django

from django.db import models

class Employee(models.Model):
    name = models.CharField(max_length=100)
    manager = models.ForeignKey('Employee', null=True, blank=True, on_delete=models.SET_NULL)

A The model must inherit from models.SelfReferencingModel

B ForeignKey cannot be null or blank in self-referencing models

C on_delete=models.SET_NULL is invalid for self-referencing fields

D The ForeignKey must use a string 'self' instead of the class name Employee

💡 Hint

Check how Django handles self-referencing ForeignKey declarations.

When referencing the same model inside its definition, Django requires the model name as a string 'self' to avoid errors.

🧠 Conceptual

expert

What is the main difference between symmetrical=True and symmetrical=False in self-referencing ManyToManyFields?

In Django, when defining a self-referencing ManyToManyField, what does setting symmetrical=True versus symmetrical=False change about the relationship?

A symmetrical=True disables the relationship; symmetrical=False enables it

B symmetrical=True means if A is related to B, then B is automatically related to A; symmetrical=False means the relationship is one-way

C symmetrical=True requires a through model; symmetrical=False does not

D symmetrical=True only works with ForeignKey; symmetrical=False only works with ManyToManyField

💡 Hint

Think about friendship versus following relationships.

symmetrical=True means the relationship is mutual (like friends). symmetrical=False means the relationship can be one-directional (like followers).