ForeignKey for one-to-many in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a ForeignKey in Django?

A ForeignKey in Django is a field used to create a one-to-many relationship between two models. It links one model to another by storing the primary key of the related model.

beginner

How do you define a one-to-many relationship using ForeignKey in Django models?

You add a ForeignKey field in the 'many' side model pointing to the 'one' side model. For example, in a Blog and Entry setup, Entry has a ForeignKey to Blog.

intermediate

What does the 'on_delete' parameter do in a ForeignKey field?

The 'on_delete' parameter defines what happens to related objects when the referenced object is deleted. Common options are CASCADE (delete related), SET_NULL (set to null), and PROTECT (prevent deletion).

intermediate

How can you access all related objects from the 'one' side in a one-to-many ForeignKey relationship?

You use the reverse relation manager. By default, Django creates a manager named <modelname>_set. For example, blog.entry_set.all() returns all entries for a blog.

beginner

Why is using ForeignKey better than storing IDs manually for relationships?

ForeignKey ensures data integrity, provides easy querying, and integrates with Django's ORM features like automatic joins and reverse lookups, making code cleaner and safer.

In Django, which model should have the ForeignKey field in a one-to-many relationship?

A The model on the 'many' side

B The model on the 'one' side

C Both models

D Neither model

The ForeignKey is placed on the 'many' side model to link it to the 'one' side model.

What does the 'on_delete=models.CASCADE' option do in a ForeignKey?

A Prevents deletion of the referenced object

B Deletes related objects when the referenced object is deleted

C Sets the ForeignKey to null on deletion

D Does nothing on deletion

CASCADE deletes all related objects automatically when the referenced object is deleted.

How do you access all related objects from the 'one' side in a ForeignKey relationship?

A You cannot access related objects from the 'one' side

B Using ForeignKey field directly

C Using <modelname>_set manager

D Using a separate query unrelated to ForeignKey

Django automatically creates a reverse manager named _set to access related objects.

Which of these is NOT a valid 'on_delete' option in Django ForeignKey?

A IGNORE

B CASCADE

C PROTECT

D SET_NULL

'IGNORE' is not a valid on_delete option in Django.

Why is using ForeignKey preferred over manually storing IDs for relationships?

A It requires more manual code

B It makes the database slower

C It disables reverse lookups

D It ensures data integrity and easier querying

ForeignKey helps maintain data integrity and provides easy querying and reverse lookups.

Explain how to create a one-to-many relationship using ForeignKey in Django models.

Think about which model holds the ForeignKey and what happens on deletion.

Describe how to access related objects from the 'one' side in a ForeignKey relationship in Django.

Remember Django creates a manager automatically for reverse lookups.

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