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

Challenge - 5 Problems

❓ component_behavior

intermediate

What does the belongs_to association do in Rails?

Consider a Rails model Comment with belongs_to :post. What behavior does this association provide?

A It sets up a connection where each comment is linked to one post, allowing access to the post via <code>comment.post</code>.

B It makes the comment model inherit all methods from the post model.

C It automatically creates a <code>posts</code> table in the database.

D It allows a comment to have many posts linked to it.

💡 Hint

Think about how one comment relates to one post in a blog.

The belongs_to association in Rails sets up a one-to-one connection where the model with belongs_to holds the foreign key. This lets you call comment.post to get the associated post.

📝 Syntax

intermediate

Identify the correct syntax for belongs_to in a Rails model

Which of the following is the correct way to declare a belongs_to association in a Rails model named OrderItem that belongs to Order?

A belongs_to order

B belongs_to :order

C belongs_to 'order'

D belongs_to :orders

💡 Hint

Look for the correct symbol usage and singular form.

The correct syntax uses a symbol for the association name in singular form: belongs_to :order. Using a string or plural form is incorrect.

🔧 Debug

advanced

Why does accessing comment.post raise an error?

Given these models:

class Comment < ApplicationRecord
belongs_to :post
end

class Post < ApplicationRecord
end

And the database has a comments table without a post_id column. What error will occur when calling comment.post and why?

A NoMethodError because <code>post</code> method is not defined in Comment.

B ActiveRecord::RecordNotFound because the post record does not exist.

C ActiveModel::MissingAttributeError because the <code>post_id</code> column is missing in the comments table.

D SyntaxError because the belongs_to association is incorrectly declared.

💡 Hint

Think about what Rails expects in the database for a belongs_to association.

The belongs_to association expects a foreign key column named post_id in the comments table. Without it, Rails cannot find the attribute and raises ActiveModel::MissingAttributeError.

❓ state_output

advanced

What is the value of comment.post.title after creation?

Given these models and code:

class Post < ApplicationRecord
has_many :comments
end

class Comment < ApplicationRecord
belongs_to :post
end

And this code runs:

post = Post.create(title: 'Hello World')
comment = Comment.create(post: post, content: 'Nice post!')

What will comment.post.title return?

A 'Nice post!'

B nil

C Raises NoMethodError

D 'Hello World'

💡 Hint

Remember the association links comment to the post object.

The comment.post returns the associated post object. Accessing title on it returns the string 'Hello World' that was set when creating the post.

🧠 Conceptual

expert

Why must the belongs_to association be declared on the model with the foreign key?

In Rails, the belongs_to association is declared on the model that holds the foreign key column. Why is this necessary?

A Because Rails uses the foreign key in that model's table to find the associated record, so the association must be declared there to work correctly.

B Because declaring it on the other model would create a circular dependency and crash the app.

C Because only the model with the foreign key can have database indexes.

D Because Rails automatically creates the foreign key column on the belongs_to model.

💡 Hint

Think about how Rails looks up the related record using the foreign key.

The belongs_to association tells Rails which foreign key column to use to find the related record. This foreign key lives in the model's own table, so the association must be declared there.