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

Concept Flow - ForeignKey for one-to-many

Define Parent Model

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Define Child Model with ForeignKey to Parent

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Create Parent Instance

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Create Child Instances linked to Parent

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Access Child Instances from Parent

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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.