ForeignKey for one-to-many in Django - Step-by-Step Execution

Concept Flow - ForeignKey for one-to-many

Define Parent Model

↓

Define Child Model with ForeignKey to Parent

↓

Create Parent Instance

↓

Create Child Instances linked to Parent

↓

Access Child Instances from Parent

↓

Use QuerySet to get all children

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Display or use related children

This flow shows how a parent model is linked to many child models using ForeignKey, then how to create and access these linked records.

Execution Sample

Django

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)

Defines Author as parent and Book as child with ForeignKey linking each book to one author.

Execution Table

Step	Action	Evaluation	Result
1	Create Author instance	Author(name='Alice')	Author object saved with id=1
2	Create Book instance linked to Author	Book(title='Book A', author=Author(id=1))	Book object saved with id=1 linked to Author id=1
3	Create another Book linked to same Author	Book(title='Book B', author=Author(id=1))	Book object saved with id=2 linked to Author id=1
4	Access books from Author	author.book_set.all()	QuerySet with Book id=1 and id=2
5	Iterate over books	for book in author.book_set.all():	Outputs titles 'Book A' and 'Book B'
6	Exit	No more books	Iteration ends

💡 All books linked to the author have been accessed and iteration ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
author	None	Author(id=1, name='Alice')	Author(id=1, name='Alice')	Author(id=1, name='Alice')	Author(id=1, name='Alice')	Author(id=1, name='Alice')
book1	None	None	Book(id=1, title='Book A', author=1)	Book(id=1, title='Book A', author=1)	Book(id=1, title='Book A', author=1)	Book(id=1, title='Book A', author=1)
book2	None	None	None	Book(id=2, title='Book B', author=1)	Book(id=2, title='Book B', author=1)	Book(id=2, title='Book B', author=1)
books_queryset	None	None	None	None	QuerySet([Book(id=1), Book(id=2)])	QuerySet([Book(id=1), Book(id=2)])

Key Moments - 2 Insights

Why do we use author.book_set.all() to get books instead of a direct attribute?

Because Django automatically creates a reverse relation named book_set for the ForeignKey. This lets us access all Book objects linked to that Author, as shown in step 4 of the execution_table.

What happens if we delete the Author instance?

Since on_delete=models.CASCADE is set, deleting the Author will also delete all linked Book instances. This is important to keep data consistent, as implied by the ForeignKey setup.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what does author.book_set.all() return?

A A single Book instance

B An empty list

C A QuerySet containing all Book instances linked to the author

D The Author instance itself

💡 Hint

Check the 'Result' column at step 4 in execution_table

At which step are multiple Book instances linked to the same Author?

A Step 3

B Step 2

C Step 1

D Step 5

💡 Hint

Look at when book2 is created linked to author in variable_tracker

If we change on_delete=models.SET_NULL instead of CASCADE, what changes in behavior?

A Books get deleted when Author is deleted

B Books remain but their author field becomes null

C Author cannot be deleted

D Nothing changes

💡 Hint

Consider the meaning of on_delete options in ForeignKey

Concept Snapshot

ForeignKey links many child records to one parent.
Define ForeignKey in child model pointing to parent.
Use parent.child_set.all() to get all children.
on_delete controls what happens if parent is deleted.
Commonly used for one-to-many relationships.

Full Transcript

This visual execution shows how Django's ForeignKey creates a one-to-many link between models. We start by defining a parent model Author and a child model Book with a ForeignKey to Author. When we create an Author instance and multiple Book instances linked to it, Django stores these relations in the database. Accessing author.book_set.all() returns all books linked to that author. The on_delete parameter controls behavior when the parent is deleted, with CASCADE deleting all linked children. This trace helps beginners see how data is connected and accessed step-by-step.

Practice

1. In Django, where should you place a ForeignKey field to represent a one-to-many relationship?

easy

A. In the model representing the 'many' side of the relationship

B. In the model representing the 'one' side of the relationship

C. In both models to link them together

D. You don't use ForeignKey for one-to-many relationships

Solution

1. Step 1: Understand the one-to-many relationship

   One object on the 'one' side can relate to many objects on the 'many' side.

2. Step 2: Place ForeignKey on the 'many' side

   The ForeignKey field goes in the model that represents the 'many' side to link back to the 'one' side.

3. Final Answer:

   In the model representing the 'many' side of the relationship -> Option A

4. Quick Check:

   ForeignKey on 'many' side = A [OK]

Hint: ForeignKey always goes on the 'many' side model [OK]

Common Mistakes:

• Putting ForeignKey on the 'one' side model
• Adding ForeignKey to both models
• Thinking ForeignKey is not used for one-to-many

2. Which of the following is the correct syntax to define a ForeignKey in a Django model named Book that links to a model named Author?

easy

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

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

C. author = models.ForeignKey('Author')

D. author = models.ForeignKey(Author, on_delete='CASCADE')

Solution

1. Step 1: Check ForeignKey syntax

   ForeignKey requires the related model and the on_delete argument to specify delete behavior.

2. Step 2: Validate correct usage

   author = models.ForeignKey(Author, on_delete=models.CASCADE) correctly uses on_delete=models.CASCADE. Options A, B, and D have syntax errors or missing required arguments.

3. Final Answer:

   author = models.ForeignKey(Author, on_delete=models.CASCADE) -> Option B

4. Quick Check:

   Correct ForeignKey syntax = C [OK]

Hint: Always include on_delete=models.CASCADE with ForeignKey [OK]

Common Mistakes:

• Omitting on_delete argument
• Using wrong argument name like delete
• Passing on_delete as string instead of constant

3. Given these 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)

What will Book.objects.filter(author__name='Alice').count() return if there are 3 books by Alice and 2 by Bob?

medium

A. 0

B. 2

C. 5

D. 3

Solution

1. Step 1: Understand the filter query

   The query filters books where the related author's name is 'Alice'.

2. Step 2: Count matching books

   Since 3 books belong to Alice, the count will be 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Books by Alice = 3 [OK]

Hint: Filter on related model with double underscore __ [OK]

Common Mistakes:

• Counting authors instead of books
• Using single underscore instead of double
• Confusing author name with book title

4. What is wrong with this Django model code?

class Comment(models.Model):
    post = models.ForeignKey(Post)
    text = models.TextField()

medium

A. TextField cannot be used for text

B. ForeignKey should be named post_id

C. Missing on_delete argument in ForeignKey

D. ForeignKey should be in Post model

Solution

1. Step 1: Check ForeignKey arguments

   Since Django 2.0, on_delete is required for ForeignKey fields.

2. Step 2: Identify missing on_delete

   The code lacks on_delete, causing an error.

3. Final Answer:

   Missing on_delete argument in ForeignKey -> Option C

4. Quick Check:

   ForeignKey requires on_delete argument [OK]

Hint: Always add on_delete to ForeignKey fields [OK]

Common Mistakes:

• Forgetting on_delete causes errors
• Renaming ForeignKey field incorrectly
• Thinking TextField is invalid for text

5. You have these models:

class Category(models.Model):
    name = models.CharField(max_length=50)

class Product(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True)

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

hard

A. Products remain but their category field is set to NULL

B. Products are deleted automatically

C. Deletion of Category is blocked if Products exist

D. Products keep the deleted category reference

Solution

1. Step 1: Understand on_delete=models.SET_NULL

   This option sets the ForeignKey field to NULL when the related object is deleted.

2. Step 2: Apply to Product-Category relation

   When a Category is deleted, related Products keep their records but their category field becomes NULL.

3. Final Answer:

   Products remain but their category field is set to NULL -> Option A

4. Quick Check:

   on_delete=SET_NULL means keep products, nullify category [OK]

Hint: SET_NULL keeps related objects, clears ForeignKey [OK]

Common Mistakes:

• Assuming related objects are deleted
• Thinking deletion is blocked
• Believing ForeignKey keeps deleted references