has_one relationship in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this has_one association output?

Given the following Rails models, what will user.profile.name output if the profile exists?

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
  # attribute :name, :string
end

user = User.create(name: 'Alice')
user.create_profile(name: 'Alice Profile')

puts user.profile.name

A Alice Profile

B nil

C Raises NoMethodError

D Raises ActiveRecord::RecordNotFound

💡 Hint

Remember that has_one creates a method to access the associated object.

The has_one association creates a method profile on User that returns the associated Profile object. Since the profile was created with name 'Alice Profile', user.profile.name returns that string.

❓ state_output

intermediate

What is the value of profile.user after creation?

Given these models and code, what will profile.user.name output?

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

user = User.create(name: 'Bob')
profile = user.create_profile

puts profile.user.name

A Raises NoMethodError

B nil

C Bob

D Raises ActiveRecord::RecordInvalid

💡 Hint

Check the belongs_to association from Profile to User.

The belongs_to association on Profile creates a method user that returns the associated User. Since the profile was created from the user, profile.user.name returns 'Bob'.

📝 Syntax

advanced

Which option correctly defines a has_one association with dependent destroy?

You want to define a has_one association where deleting the parent deletes the child. Which code is correct?

A has_one :profile, dependent: destroy

B has_one :profile, dependent: :destroy

C has_one :profile, dependent => :destroy

D has_one :profile, dependent: 'destroy'

💡 Hint

Check the syntax for passing options as symbols.

The correct syntax uses a symbol for the option value: dependent: :destroy. Option B uses a string instead of a symbol, B misses the colon before destroy, and C uses the older hash rocket syntax incorrectly.

🔧 Debug

advanced

Why does user.profile return nil unexpectedly?

Given these models and code, why does user.profile return nil even after creating a profile?

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

user = User.create(name: 'Carol')
Profile.create(name: 'Carol Profile')

puts user.profile

A has_one association requires reload to update, so user.profile is stale

B User must use has_many for multiple profiles, so has_one returns nil

C belongs_to is missing in Profile, so association fails

D Profile was created without linking to the user, so user.profile is nil

💡 Hint

Check if the profile is linked to the user when created.

The profile was created without specifying the user_id, so it is not linked to the user. Therefore, user.profile returns nil.

🧠 Conceptual

expert

What happens if you call build_profile twice on the same user?

Consider a user with no profile. You call user.build_profile(name: 'First') then user.build_profile(name: 'Second') without saving. What is the state of user.profile.name?

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

user = User.create(name: 'Dave')
user.build_profile(name: 'First')
user.build_profile(name: 'Second')

puts user.profile.name

A Second

B First

C nil

D Raises ActiveRecord::RecordNotSaved

💡 Hint

Think about what build_profile does and if it replaces the previous unsaved profile.

Calling build_profile twice replaces the unsaved profile object. The last built profile is returned by user.profile, so the name is 'Second'.