Scopes for reusable queries in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this scope return?

Given the following Rails model scope, what records will User.active return?

Ruby on Rails

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
end

users = User.active

A All users regardless of their active status

B All users with active set to false

C An error because the scope is not defined correctly

D All users with the attribute active set to true

💡 Hint

Think about what the where clause filters.

The scope active filters users where the active attribute is true, so User.active returns only those users.

📝 Syntax

intermediate

Identify the syntax error in this scope definition

Which option shows the correct way to define a scope that returns users created in the last 7 days?

Ruby on Rails

class User < ApplicationRecord
  scope :recent, -> { where('created_at >= ?', 7.days.ago) }
end

A scope :recent, -> { where('created_at >= ?', 7.days.ago) }

B scope :recent, -> { where(created_at >= 7.days.ago) }

C scope :recent, -> { where('created_at >= 7.days.ago') }

D scope :recent, -> { where(created_at: 7.days.ago) }

💡 Hint

Check how to pass dynamic values in SQL conditions.

Option A correctly uses a SQL string with a placeholder and passes the dynamic value as a parameter. Other options either miss quotes or pass Ruby objects incorrectly.

❓ state_output

advanced

What is the output count of this chained scope?

Given these scopes, what is the count of User.active.recent.count if there are 5 active users, 3 users created in last 7 days, and 2 users who are both active and recent?

Ruby on Rails

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
  scope :recent, -> { where('created_at >= ?', 7.days.ago) }
end

count = User.active.recent.count

A 8

B 2

C 5

D 3

💡 Hint

Chaining scopes combines their conditions with AND logic.

Chaining active and recent scopes returns users who are both active and recent. Only 2 users meet both conditions.

🔧 Debug

advanced

Why does this scope raise an error?

This scope is intended to return users with a minimum age, but it raises an error. Why?

Ruby on Rails

class User < ApplicationRecord
  scope :min_age, ->(age) { where('age >= age') }
end

User.min_age(18)

A The scope should use a hash instead of a string condition

B The scope is missing a block parameter

C The SQL condition uses a string 'age >= age' instead of a placeholder for the parameter

D The scope is missing a call to .to_i on age

💡 Hint

Look at how the parameter is used inside the where clause.

The condition 'age >= age' compares the column to itself, not to the passed parameter. It should use a placeholder 'age >= ?' and pass age as argument.

🧠 Conceptual

expert

How do scopes improve code reuse and readability in Rails?

Which option best explains the main benefit of using scopes for reusable queries in Rails?

A Scopes allow defining common query logic once and reuse it easily, making code cleaner and easier to maintain

B Scopes automatically cache query results to improve performance without extra code

C Scopes replace the need for database indexes by optimizing queries internally

D Scopes force all queries to be eager loaded to avoid N+1 problems

💡 Hint

Think about how scopes help avoid repeating the same query code.

Scopes let you write query logic once and reuse it by calling the scope method. This keeps code DRY and easier to read and maintain.