has_one relationship in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a has_one association in a Rails model.

Ruby on Rails

class User < ApplicationRecord
  [1] :profile
end

Drag options to blanks, or click blank then click option'

Abelongs_to

Bhas_one

Chas_many

Dhas_and_belongs_to_many

Attempts:

3 left

💡 Hint

Common Mistakes

Using belongs_to instead of has_one here.

Using has_many which is for multiple related objects.

The has_one method declares a one-to-one association from the User model to the Profile model.

2fill in blank

medium

Complete the code to declare the inverse belongs_to association in the related model.

Ruby on Rails

class Profile < ApplicationRecord
  [1] :user
end

Drag options to blanks, or click blank then click option'

Ahas_one

Bhas_many

Cbelongs_to

Dhas_and_belongs_to_many

Attempts:

3 left

💡 Hint

Common Mistakes

Using has_one here instead of belongs_to.

Using has_many which is for multiple related objects.

The belongs_to method declares the association from Profile back to User, indicating Profile belongs to one User.

3fill in blank

hard

Fix the error in the has_one declaration to specify the class name correctly.

Ruby on Rails

class User < ApplicationRecord
  has_one :[1], class_name: 'UserProfile'
end

Drag options to blanks, or click blank then click option'

Auserprofiles

Bprofiles

Cuser_profile

Dprofile

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural form like 'profiles' for has_one.

Using underscored or concatenated names that don't match the model.

The association name should be singular and match the related model logically. 'profile' is correct here.

4fill in blank

hard

Fill both blanks to complete the has_one association with dependent destroy option.

Ruby on Rails

class User < ApplicationRecord
  [1] :profile, [2]: :destroy
end

Drag options to blanks, or click blank then click option'

Ahas_one

Bdependent

Cbelongs_to

Doptional

Attempts:

3 left

💡 Hint

Common Mistakes

Using belongs_to instead of has_one for the association.

Using optional instead of dependent for the option.

has_one declares the association, and dependent: :destroy ensures the related profile is deleted when the user is deleted.

5fill in blank

hard

Fill all three blanks to complete a has_one association with class name and foreign key options.

Ruby on Rails

class User < ApplicationRecord
  [1] :profile, class_name: '[2]', foreign_key: '[3]'
end

Drag options to blanks, or click blank then click option'

Ahas_one

BUserProfile

Cuser_id

Dbelongs_to

Attempts:

3 left

💡 Hint

Common Mistakes

Using belongs_to instead of has_one for the association.

Incorrect class name spelling or casing.

Wrong foreign key name that doesn't match the database column.

has_one declares the association. class_name specifies the related model name. foreign_key tells Rails which column links the models.

Practice

1. What does the has_one association in Rails represent?

easy

A. A many-to-many connection between two models

B. A one-to-many connection from one model to many others

C. A one-to-one connection from one model to another

D. A model inheriting from another model

Solution

1. Step 1: Understand the purpose of has_one

   The has_one association sets up a one-to-one link from one model to another, meaning each instance of the first model is connected to exactly one instance of the second model.

2. Step 2: Compare with other associations

   Unlike has_many (one-to-many) or has_and_belongs_to_many (many-to-many), has_one is strictly one-to-one.

3. Final Answer:

   A one-to-one connection from one model to another -> Option C

4. Quick Check:

   has_one = one-to-one [OK]

Hint: Remember: has_one means exactly one related record [OK]

Common Mistakes:

• Confusing has_one with has_many
• Thinking has_one creates many related records
• Mixing up has_one with belongs_to

2. Which of the following is the correct syntax to declare a has_one association in a Rails model?

easy

A. has_one = :profile

B. has_one profile

C. has_one => :profile

D. has_one :profile

Solution

1. Step 1: Recall Rails association syntax

   In Rails, associations are declared using the method name followed by a symbol representing the related model, e.g., has_one :profile.

2. Step 2: Identify correct Ruby syntax

   has_one :profile uses the correct Ruby method call with a symbol argument. Options A, B, and C are invalid Ruby syntax for this purpose.

3. Final Answer:

   has_one :profile -> Option D

4. Quick Check:

   Association syntax uses symbol with colon [OK]

Hint: Use colon before association name: has_one :name [OK]

Common Mistakes:

• Omitting the colon before the association name
• Using hash rocket syntax incorrectly
• Assigning association with equals sign

3. Given these models:

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

What will user.profile return if the user has no profile record?

medium

A. nil

B. An error is raised

C. An empty Profile object

D. A new unsaved Profile instance

Solution

1. Step 1: Understand has_one association behavior

   If no associated record exists, calling user.profile returns nil because there is no linked profile.

2. Step 2: Confirm no automatic creation

   Rails does not create or return empty or new instances automatically for has_one associations.

3. Final Answer:

   nil -> Option A

4. Quick Check:

   No associated record means nil [OK]

Hint: No linked record? Association returns nil [OK]

Common Mistakes:

• Expecting an empty or new object instead of nil
• Assuming an error is raised when missing association
• Confusing with build_profile method

4. Identify the error in this code snippet:

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  has_one :user
end

medium

A. Profile should use belongs_to :user instead of has_one

B. User should use belongs_to :profile instead of has_one

C. Both models should use has_one for the association

D. No error; the code is correct

Solution

1. Step 1: Understand correct association directions

   In a one-to-one relationship, the model holding the foreign key uses belongs_to. Here, Profile should belong to User.

2. Step 2: Identify incorrect association

   Using has_one :user in Profile is wrong because it implies Profile owns User, which is not the case.

3. Final Answer:

   Profile should use belongs_to :user instead of has_one -> Option A

4. Quick Check:

   Foreign key side uses belongs_to [OK]

Hint: belongs_to goes on model with foreign key [OK]

Common Mistakes:

• Using has_one on both sides
• Confusing which model holds foreign key
• Omitting belongs_to in related model

5. You want to ensure that when a User is deleted, their associated Profile is also deleted automatically. How do you modify the has_one association in the User model?

hard

A. has_one :profile, autosave: true

B. has_one :profile, dependent: :destroy

C. has_one :profile, validate: true

D. has_one :profile, inverse_of: :user

Solution

1. Step 1: Understand dependent option in associations

   The dependent: :destroy option tells Rails to delete the associated record when the owner is deleted.

2. Step 2: Apply to has_one association

   Adding dependent: :destroy to has_one :profile ensures the profile is removed automatically with the user.

3. Final Answer:

   has_one :profile, dependent: :destroy -> Option B

4. Quick Check:

   dependent: :destroy deletes associated record [OK]

Hint: Use dependent: :destroy to auto-delete associated record [OK]

Common Mistakes:

• Using autosave or validate instead of dependent
• Forgetting to add dependent option
• Confusing inverse_of with dependent