Related name for reverse access in Django - Mini Project: Build & Apply

Using Related Name for Reverse Access in Django Models

📖 Scenario: You are building a simple blog application where each Author can have multiple Post entries. You want to access all posts written by an author easily from the author object.

🎯 Goal: Create two Django models, Author and Post, where Post has a foreign key to Author with a related_name set. This will allow reverse access from an author to their posts using the related name.

📋 What You'll Learn

Create a Django model called Author with a name field

Create a Django model called Post with a title field

Add a foreign key in Post to Author with related_name='posts'

Use the related_name to access all posts of an author

💡 Why This Matters

🌍 Real World

In many web applications, you need to model relationships between data, such as authors and their posts. Using related_name makes it easy to navigate these relationships in your code.

💼 Career

Understanding Django model relationships and reverse access is essential for backend developers working with Django to build scalable and maintainable web applications.

1

Create the Author model

Create a Django model called Author with a single field name which is a CharField with max length 100.

Django

# Your code here

Hint

Use models.CharField(max_length=100) for the name field.

2

Create the Post model with foreign key to Author

Create a Django model called Post with a title field as CharField max length 200, and add a foreign key field called author to the Author model with related_name='posts'.

Django

from django.db import models

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

# Your code here

Hint

Use models.ForeignKey with on_delete=models.CASCADE and related_name='posts'.

3

Access posts from an author using related_name

Write a Django ORM query to get all posts of an Author instance stored in variable author using the related_name posts. Assign the result to a variable called author_posts.

Django

from django.db import models

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

class Post(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='posts')

# Assume author is an Author instance
# Your code here

Hint

Use the related_name posts on the author instance to get all posts.

4

Add __str__ methods for better display

Add a __str__ method to both Author and Post models that return the name and title fields respectively.

Django

from django.db import models

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

class Post(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='posts')

# Assume author is an Author instance
author_posts = author.posts.all()

# Your code here

Hint

Define __str__ methods returning the main identifying field for each model.

Practice

1. What does the related_name attribute do in a Django model's ForeignKey field?

easy

A. It sets the name used to access related objects from the other model.

B. It changes the database table name for the model.

C. It defines the primary key of the model.

D. It specifies the default ordering of query results.

Solution

1. Step 1: Understand the purpose of related_name

   The related_name attribute defines how you access related objects from the reverse side of a ForeignKey relationship.

2. Step 2: Identify what related_name affects

   It does not affect table names, primary keys, or ordering but sets the attribute name for reverse access.

3. Final Answer:

   It sets the name used to access related objects from the other model. -> Option A

4. Quick Check:

   related_name = reverse access name [OK]

Hint: related_name names reverse access from related model [OK]

Common Mistakes:

• Confusing related_name with table name
• Thinking it sets primary key
• Assuming it controls query ordering

2. Which of the following is the correct way to set a related_name in a Django ForeignKey field?

easy

A. author = models.ForeignKey(Author, reverse_name='books', on_delete=models.CASCADE)

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

C. author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE)

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

Solution

1. Step 1: Recall correct attribute name

   The correct attribute to set reverse access name is related_name.

2. Step 2: Check syntax correctness

   author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE) uses related_name='books' correctly; others use incorrect attribute names.

3. Final Answer:

   author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE) -> Option C

4. Quick Check:

   Use related_name= for reverse access [OK]

Hint: Use exact attribute name 'related_name' in ForeignKey [OK]

Common Mistakes:

• Using 'related' instead of 'related_name'
• Using 'reverse_name' or 'relation_name' which don't exist
• Missing on_delete argument

3. Given these models:

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

class Book(models.Model):
    author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE)
    title = models.CharField(max_length=100)

What will author.books.all() return?

medium

A. All Author objects related to that Book instance.

B. All Book objects related to that Author instance.

C. A syntax error because 'books' is not defined.

D. An empty queryset always.

Solution

1. Step 1: Understand related_name usage

   The related_name='books' allows accessing all Book objects from an Author instance using author.books.

2. Step 2: Interpret the method call

   author.books.all() returns a queryset of all Book objects linked to that Author.

3. Final Answer:

   All Book objects related to that Author instance. -> Option B

4. Quick Check:

   author.books.all() = related books [OK]

Hint: related_name lets you get all related objects easily [OK]

Common Mistakes:

• Thinking it returns Author objects
• Assuming syntax error due to related_name
• Expecting empty queryset without data

4. Identify the error in this model definition:

class Comment(models.Model):
    post = models.ForeignKey(Post, related_name='post', on_delete=models.CASCADE)
    text = models.TextField()

What problem will this cause?

medium

A. It will cause a syntax error because related_name cannot be 'post'.

B. It will cause a runtime error because TextField is not allowed here.

C. It will work fine without any issues.

D. It will cause a reverse accessor clash if Post already has a field named 'post'.

Solution

1. Step 1: Understand related_name uniqueness

   The related_name must be unique per model to avoid clashes.

2. Step 2: Analyze the name 'post'

   If Post model already has a field or reverse accessor named 'post', this causes a clash error.

3. Final Answer:

   It will cause a reverse accessor clash if Post already has a field named 'post'. -> Option D

4. Quick Check:

   related_name must be unique to avoid clashes [OK]

Hint: Avoid related_name same as model or field names [OK]

Common Mistakes:

• Assuming related_name can be any string without conflict
• Thinking related_name causes syntax error
• Believing TextField is invalid here

5. You have these models:

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

class Product(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE)

You want to access all products of a category using category.items.all(). How should you modify the ForeignKey?

hard

A. Add related_name='items' to the ForeignKey in Product.

B. Change ForeignKey to ManyToManyField with related_name='items'.

C. Rename the ForeignKey field to 'items'.

D. Add related_query_name='items' to the ForeignKey.

Solution

1. Step 1: Understand reverse access naming

   To use category.items.all(), the ForeignKey must have related_name='items'.

2. Step 2: Check other options

   Changing to ManyToManyField or renaming the field won't create the desired reverse attribute. related_query_name affects query filters, not attribute names.

3. Final Answer:

   Add related_name='items' to the ForeignKey in Product. -> Option A

4. Quick Check:

   related_name sets reverse attribute name [OK]

Hint: Set related_name='items' to get category.items [OK]

Common Mistakes:

• Confusing related_name with related_query_name
• Changing field name instead of related_name
• Switching to ManyToManyField unnecessarily