F expressions for field comparisons in Django - Mini Project: Build & Apply

F expressions for field comparisons

📖 Scenario: You are building a Django app to manage a bookstore. You want to find all books where the number of copies sold is greater than the number of copies in stock.

🎯 Goal: Create a Django query using F expressions to compare two fields in the Book model and filter books where copies_sold is greater than copies_in_stock.

📋 What You'll Learn

Create a Django model called Book with fields title (CharField), copies_sold (IntegerField), and copies_in_stock (IntegerField).

Create a variable threshold set to 0 (to use later).

Use Django's F expression to filter Book objects where copies_sold is greater than copies_in_stock.

Assign the filtered queryset to a variable called books_sold_more_than_stock.

💡 Why This Matters

🌍 Real World

Comparing fields in database records is common in apps like inventory management, sales tracking, and more.

💼 Career

Understanding F expressions helps you write efficient database queries in Django, a key skill for backend developers.

1

Create the Book model

Create a Django model called Book with fields: title as a CharField with max length 100, copies_sold as an IntegerField, and copies_in_stock as an IntegerField.

Django

# Define the Book model with the specified fields
# Your code here

Need a hint?

Use models.CharField for text and models.IntegerField for numbers.

2

Add a threshold variable

Create a variable called threshold and set it to 0.

Django

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=100)
    copies_sold = models.IntegerField()
    copies_in_stock = models.IntegerField()

# Create threshold variable
# Your code here

Need a hint?

Just assign 0 to a variable named threshold.

3

Filter books using F expressions

Import F from django.db.models. Use Book.objects.filter() with an F expression to filter books where copies_sold is greater than copies_in_stock. Assign the result to books_sold_more_than_stock.

Django

from django.db import models
from django.db.models import F

class Book(models.Model):
    title = models.CharField(max_length=100)
    copies_sold = models.IntegerField()
    copies_in_stock = models.IntegerField()

threshold = 0

# Filter books where copies_sold > copies_in_stock
# Your code here

Need a hint?

Use copies_sold__gt=F('copies_in_stock') inside filter().

4

Complete the Django query setup

Ensure the import of F from django.db.models is present and the variable books_sold_more_than_stock holds the filtered queryset where copies_sold is greater than copies_in_stock.

Django

from django.db import models
from django.db.models import F

class Book(models.Model):
    title = models.CharField(max_length=100)
    copies_sold = models.IntegerField()
    copies_in_stock = models.IntegerField()

threshold = 0

books_sold_more_than_stock = Book.objects.filter(copies_sold__gt=F('copies_in_stock'))

# Finalize the code if needed
# Your code here

Need a hint?

Check that the import and filter line are exactly as specified.