F expressions for field comparisons in Django - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import F expressions from Django.

Django

from django.db.models import [1]

Drag options to blanks, or click blank then click option'

AF

BValue

CQ

DCase

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Q instead of F

Forgetting to import F

Importing unrelated classes like Value or Case

The F class is imported from django.db.models to allow field comparisons in queries.

2fill in blank

medium

Complete the code to filter objects where 'price' is greater than 'cost' using F expressions.

Django

Product.objects.filter(price__[1]=F('cost'))

Drag options to blanks, or click blank then click option'

Agt

Blt

Clte

Dgte

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'lt' which means less than

Using 'gte' or 'lte' which include equality

Using exact which means equal

The lookup gt means 'greater than'. This filters products where price is greater than cost.

3fill in blank

hard

Fix the error in the code to compare 'quantity' with 'minimum_quantity' using F expressions.

Django

Inventory.objects.filter(quantity__[1]=F('minimum_quantity'))

Drag options to blanks, or click blank then click option'

Aequals

Beq

Cis

Dexact

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'eq' or 'equals' which are not valid lookups

Using 'is' which is a Python keyword, not a lookup

The correct lookup for equality in Django queries is exact. 'eq', 'equals', and 'is' are invalid lookups.

4fill in blank

hard

Fill both blanks to update 'stock' by subtracting 'sold' using F expressions.

Django

Product.objects.update(stock=F('stock') [1] F('[2]'))

Drag options to blanks, or click blank then click option'

A-

B+

Csold

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causing stock to increase

Using wrong field name like 'price' instead of 'sold'

To reduce stock by the sold amount, subtract 'sold' from 'stock' using the '-' operator and the field name 'sold'.

5fill in blank

hard

Fill all three blanks to filter orders where 'total' is at least twice the 'cost'.

Django

Order.objects.filter(total__[1]=F('cost') [2] [3])

Drag options to blanks, or click blank then click option'

Agte

B*

C2

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' for multiplication

Using 'gt' instead of 'gte' to exclude equal values

Using wrong number instead of 2

Use 'gte' for 'greater than or equal', multiply 'cost' by 2 using '* 2' to compare with 'total'.

Practice

1. What is the main purpose of using F expressions in Django ORM?

easy

A. To compare or update model fields directly in the database without fetching data

B. To convert query results into Python dictionaries

C. To create new database tables automatically

D. To write raw SQL queries inside Django models

Solution

1. Step 1: Understand what F expressions do

   F expressions allow referencing model fields directly in queries without loading data into Python.

2. Step 2: Identify the correct use case

   This lets you compare or update fields efficiently in the database, avoiding extra data transfer.

3. Final Answer:

   To compare or update model fields directly in the database without fetching data -> Option A

4. Quick Check:

   F expressions = direct DB field operations [OK]

Hint: F expressions work inside queries without loading data [OK]

Common Mistakes:

• Thinking F expressions convert results to dicts
• Confusing F expressions with migrations
• Assuming F expressions run raw SQL

2. Which of the following is the correct syntax to filter objects where the field score is greater than the field min_score using F expressions?

easy

A. Model.objects.filter(F('score') > F('min_score'))

B. Model.objects.filter(score > F('min_score'))

C. Model.objects.filter(score__gt='min_score')

D. Model.objects.filter(score__gt=F('min_score'))

Solution

1. Step 1: Recall correct filter syntax with F expressions

   Use field lookups like score__gt=F('min_score') to compare fields.

2. Step 2: Check each option

   Model.objects.filter(score__gt=F('min_score')) uses correct Django ORM syntax. Model.objects.filter(score > F('min_score')) uses invalid Python syntax inside filter. Model.objects.filter(score__gt='min_score') compares to string, not field. Model.objects.filter(F('score') > F('min_score')) is invalid syntax.

3. Final Answer:

   Model.objects.filter(score__gt=F('min_score')) -> Option D

4. Quick Check:

   Use field lookups with F('field') [OK]

Hint: Use field lookups like __gt with F('field') [OK]

Common Mistakes:

• Using Python operators inside filter()
• Passing field names as strings instead of F expressions
• Confusing field lookup syntax

3. Given the model Product with fields price and discount_price, what will this query return?

Product.objects.filter(discount_price__lt=F('price')).count()

medium

A. The total number of products in the database

B. The number of products where discount_price is less than price

C. The number of products where discount_price equals price

D. Raises a syntax error

Solution

1. Step 1: Understand the filter condition

   The filter selects products where discount_price is less than price using an F expression.

2. Step 2: Understand the count() method

   It returns the number of records matching the filter condition.

3. Final Answer:

   The number of products where discount_price is less than price -> Option B

4. Quick Check:

   filter with F expression returns matching count [OK]

Hint: F expressions compare fields inside filters correctly [OK]

Common Mistakes:

• Thinking count() returns all products
• Confusing less than with equals
• Assuming syntax error due to F expression

4. Identify the error in this Django query using F expressions:

Order.objects.filter(total__gt=F(total_paid))

medium

A. Missing quotes around the field name in F expression

B. Using __gt instead of __lt for comparison

C. F expressions cannot be used in filters

D. total and total_paid fields must be integers

Solution

1. Step 1: Check F expression syntax

   The field name inside F() must be a string, so it should be F('total_paid').

2. Step 2: Analyze the given query

   The query uses F(total_paid) without quotes, causing a NameError or syntax error.

3. Final Answer:

   Missing quotes around the field name in F expression -> Option A

4. Quick Check:

   F('field_name') requires quotes [OK]

Hint: Always put field names as strings inside F() [OK]

Common Mistakes:

• Omitting quotes inside F()
• Confusing comparison operators
• Believing F expressions can't be in filters

5. You want to update all Employee records to increase their salary by the value in their bonus field using F expressions. Which code snippet correctly performs this update?

hard

A. Employee.objects.update(salary=F('salary' + 'bonus'))

B. Employee.objects.update(salary='salary + bonus')

C. Employee.objects.update(salary=F('salary') + F('bonus'))

D. Employee.objects.update(salary=F('salary') - F('bonus'))

Solution

1. Step 1: Understand how to update fields with F expressions

   You can perform arithmetic operations between fields using F expressions like F('salary') + F('bonus').

2. Step 2: Check each option

   Employee.objects.update(salary=F('salary') + F('bonus')) correctly adds the two fields. Employee.objects.update(salary=F('salary' + 'bonus')) incorrectly concatenates strings inside F(). Employee.objects.update(salary='salary + bonus') assigns a string, not a field operation. Employee.objects.update(salary=F('salary') - F('bonus')) subtracts instead of adding.

3. Final Answer:

   Employee.objects.update(salary=F('salary') + F('bonus')) -> Option C

4. Quick Check:

   Use arithmetic with F('field') for updates [OK]

Hint: Use F('field1') + F('field2') for field arithmetic updates [OK]

Common Mistakes:

• Concatenating field names as strings inside F()
• Assigning string expressions instead of F expressions
• Using wrong arithmetic operator