Query interface basics in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this ActiveRecord query?

Given a Rails model User with records having age attributes, what does this query return?

User.where(age: 20..30).pluck(:name)

A An array of user ages between 20 and 30

B An array of all user names regardless of age

C An array of names of users whose age is between 20 and 30 inclusive

D A hash with user names as keys and ages as values for users aged 20 to 30

💡 Hint

Think about what where and pluck do in ActiveRecord.

The where(age: 20..30) filters users with age between 20 and 30 inclusive. The pluck(:name) extracts only the name attribute values from those filtered records, returning an array of names.

📝 Syntax

intermediate

Which option correctly finds users created after 2020-01-01?

Select the correct ActiveRecord query to find users created after January 1, 2020.

A User.where('created_at > ?', '2020-01-01')

B User.where(created_at: '> 2020-01-01')

C User.where(created_at > '2020-01-01')

D User.where(created_at: ['2020-01-01'..])

💡 Hint

Remember how to use raw SQL conditions with placeholders in where.

Option A uses a raw SQL condition with a placeholder, which is the correct syntax. Option A is invalid syntax for range or comparison. Option A is invalid Ruby syntax. Option A uses an invalid range syntax in a hash.

❓ state_output

advanced

What is the count of users returned by this query?

Assuming the users table has 10 records with active true and 5 with false, what does this return?

User.where(active: true).limit(3).count

A 5

B 10

C 15

D 3

💡 Hint

Remember how count works with limit in ActiveRecord.

The count method respects limit and counts only the limited number of records. So it counts 3 active users, not all 10.

🔧 Debug

advanced

Why does this query raise an error?

Given the query:

User.where(name: 'Alice').order(:nonexistent_column)

Why does it raise an error?

A Because <code>nonexistent_column</code> does not exist in the users table

B Because <code>name</code> is not a valid attribute

C Because <code>order</code> cannot be used after <code>where</code>

D Because <code>where</code> requires a block

💡 Hint

Check the column names in the database schema.

The error occurs because order tries to sort by a column that does not exist in the database table, causing a database error.

🧠 Conceptual

expert

Which option best describes lazy loading in ActiveRecord queries?

Select the statement that correctly explains lazy loading behavior in ActiveRecord query interface.

A Queries are executed in the background asynchronously by default

B Queries are executed only when the data is actually needed, like when iterating or accessing records

C Queries are never executed automatically and must be manually triggered

D Queries are executed immediately when the query method is called

💡 Hint

Think about when ActiveRecord fetches data from the database.

ActiveRecord builds query objects lazily. The actual database query runs only when the data is accessed, such as by iterating or calling methods like to_a.