Active Record pattern in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Active Record query?

Given a Rails model Book with records:

Book.create(title: 'Ruby Basics', pages: 150)
Book.create(title: 'Rails Advanced', pages: 300)
Book.create(title: 'JavaScript Guide', pages: 200)

What does Book.where('pages > ?', 180).pluck(:title) return?

A ['Rails Advanced', 'JavaScript Guide']

B ['Ruby Basics', 'Rails Advanced']

C ['Ruby Basics', 'JavaScript Guide']

D ['Rails Advanced']

💡 Hint

Think about which books have more than 180 pages.

The query filters books with pages greater than 180, which are 'Rails Advanced' (300 pages) and 'JavaScript Guide' (200 pages). The pluck(:title) returns their titles as an array.

❓ lifecycle

intermediate

Which callback runs before saving a record?

In Rails Active Record, which callback method is called before a record is saved to the database?

A after_create

B before_create

C before_save

D after_save

💡 Hint

It runs before both creating and updating the record.

The before_save callback runs before both creating a new record and updating an existing one. before_create runs only before creating a new record.

📝 Syntax

advanced

What error does this Active Record code raise?

Consider this code snippet:

user = User.find_by(email: '[e-mail]')
user.update!(name: nil)

Assuming the name field has a NOT NULL database constraint, what error will this code raise?

A ActiveRecord::StatementInvalid

B ActiveRecord::RecordNotFound

C ActiveRecord::RecordInvalid

D No error, update succeeds

💡 Hint

Think about what happens when the database rejects a null value for a NOT NULL column.

The update! method raises an exception if the database rejects the update. Since name cannot be null, the database raises an error, which Active Record wraps as ActiveRecord::StatementInvalid.

❓ state_output

advanced

What is the value of user.persisted? after user.destroy?

Given:

user = User.create(name: 'Alice')
user.destroy

What does user.persisted? return after the destroy call?

A true

B false

C nil

D Raises an error

💡 Hint

Think about whether the object still exists in the database after destroy.

After calling destroy, the record is removed from the database, so persisted? returns false indicating it is no longer saved.

🔧 Debug

expert

Why does this Active Record query raise an error?

Examine this code:

Order.where(status: 'pending').find(1)

Assuming there is an order with ID 1 but its status is 'completed', why does this code raise an error?

A No error, returns the order with ID 1

B ActiveRecord::StatementInvalid due to invalid SQL syntax

C ActiveRecord::RecordInvalid because status is invalid

D ActiveRecord::RecordNotFound because the order with ID 1 does not match the status condition

💡 Hint

Remember that find respects scopes and where filters the query.

The where scopes the query to only orders with status 'pending'. The find(1) looks for ID 1 within that scope. Since order 1 has status 'completed', it is not found, raising ActiveRecord::RecordNotFound.