Select and pluck in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this ActiveRecord query return?

Consider a Rails model User with attributes id, name, and age. What is the output of this code?

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

Ruby on Rails

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

A A single string with all names concatenated for users aged 30

B An array of User objects with age 30

C A hash with keys as user ids and values as names of users aged 30

D An array of names of users who are 30 years old

💡 Hint

Remember that pluck extracts specific column values as an array.

The where filters users by age 30, and pluck(:name) returns an array of their names only, not full objects or hashes.

📝 Syntax

intermediate

Which option correctly uses select and pluck together?

You want to get the names of users older than 25. Which code snippet is correct?

A User.select(:name).where('age > ?', 25).pluck(:name)

B User.where('age > ?', 25).select(:name).pluck(:name)

C User.pluck(:name).select { |name| name.age > 25 }

D User.where('age > ?', 25).pluck(:name).select(:name)

💡 Hint

Think about the order of filtering and selecting columns.

where filters first, then select limits columns, and pluck extracts the column values. Option B follows this order correctly.

🔧 Debug

advanced

Why does this code raise an error?

Given the code:

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

Why does it raise an error?

Ruby on Rails

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

A <code>pluck</code> returns an array, which does not support <code>where</code>

B <code>where</code> must come before <code>pluck</code> because <code>pluck</code> returns a relation

C The syntax is correct; the error is from missing parentheses

D <code>pluck</code> can only be used with <code>select</code> first

💡 Hint

Check what type pluck returns and what methods are available on it.

pluck returns a plain Ruby array, which does not have ActiveRecord query methods like where. So calling where after pluck causes a NoMethodError.

❓ state_output

advanced

What is the output of this chained query?

Given the following code:

User.where(active: true).select(:id, :name).pluck(:name)

What does it return?

Ruby on Rails

User.where(active: true).select(:id, :name).pluck(:name)

A An array of names of active users

B An array of hashes with id and name of active users

C An array of User objects with only id and name loaded

D A single string concatenating all active user names

💡 Hint

Remember what pluck returns after selecting columns.

The where filters active users, select limits columns but pluck(:name) extracts only the names as an array of strings.

🧠 Conceptual

expert

Why prefer pluck over select when fetching specific columns?

In Rails, when fetching only one or two columns from a large table, why might pluck be more efficient than select?

A <code>select</code> returns arrays while <code>pluck</code> returns ActiveRecord objects, making <code>pluck</code> faster

B <code>select</code> always loads all columns regardless of arguments, so it is slower

C <code>pluck</code> fetches only the requested columns directly from the database and returns plain Ruby arrays, avoiding object instantiation

D <code>pluck</code> caches the results automatically, improving performance

💡 Hint

Think about what objects are created and how much data is loaded.

pluck directly retrieves the specified columns as arrays from the database, skipping the creation of ActiveRecord objects, which makes it faster and uses less memory.