F expressions for field comparisons in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Django query return?

Consider a Django model Product with fields price and discount_price. What does this query return?

Product.objects.filter(price__gt=F('discount_price'))

A All products where the price is greater than the discount price.

B All products where the price is less than the discount price.

C All products where the price equals the discount price.

D All products regardless of price or discount price.

💡 Hint

Remember that F('discount_price') refers to the value of the discount_price field in the database.

The filter uses an F expression to compare the price field to the discount_price field for each product. It returns products where price is greater than discount_price.

📝 Syntax

intermediate

Which option correctly uses an F expression to find records where two fields are equal?

You want to find all Order objects where total_price equals amount_paid. Which query is correct?

A Order.objects.filter(total_price=F('amount_paid'))

B Order.objects.filter(total_price__eq=F('amount_paid'))

C Order.objects.filter(F('total_price') == F('amount_paid'))

D Order.objects.filter(total_price=amount_paid)

💡 Hint

Use the correct Django ORM syntax for field comparisons with F expressions.

Option A correctly uses the Django ORM syntax to compare two fields using an F expression. Option A uses an invalid lookup, C uses Python syntax not supported in filters, and D compares to a variable, not a field.

🔧 Debug

advanced

Why does this Django query raise an error?

Given the model Employee with fields salary and bonus, what causes the error in this query?

Employee.objects.filter(salary > F('bonus'))

A F expressions cannot be used with numeric fields.

B The field names salary and bonus must be strings inside F expressions.

C The filter method requires a list, not a comparison.

D The comparison operator > cannot be used directly inside filter; use lookup syntax instead.

💡 Hint

Check how Django expects comparisons in filter arguments.

Django ORM filters require lookups like __gt for greater than comparisons. Using > directly causes a syntax error.

❓ state_output

advanced

What is the result count of this query?

Assume the Book model has fields pages and read_pages. Which query returns books where pages are at least twice the read_pages?

Book.objects.filter(pages__gte=F('read_pages') * 2)

What does this query return?

A Books where pages equal read_pages multiplied by 2.

B Books where the read pages are greater than or equal to twice the total pages.

C Books where the total pages are greater than or equal to twice the read pages.

D Books where pages are less than twice the read pages.

💡 Hint

Remember that __gte means greater than or equal to.

The filter uses an F expression with multiplication to compare pages to twice read_pages, returning books where pages >= 2 * read_pages.

🧠 Conceptual

expert

What happens when you use F expressions in update() with field comparisons?

Consider this code:

Product.objects.filter(stock__lt=F('reorder_level')).update(stock=F('stock') + 10)

What is the effect of this update?

A It decreases stock by 10 for products where stock is less than reorder_level.

B It increases the stock by 10 only for products where stock is less than reorder_level.

C It sets stock to 10 for all products regardless of stock or reorder_level.

D It causes a runtime error because F expressions cannot be used in update().

💡 Hint

Think about how update() and F expressions work together in Django.

The update uses an F expression to add 10 to the stock field only for products where stock is less than reorder_level, modifying those records in the database.