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.

Practice

1. What does the belongs_to association in Rails indicate about the relationship between two models?

easy

A. It creates a many-to-many relationship between two models.

B. The model with belongs_to is the parent and links to many child models.

C. It automatically creates a database table for the association.

D. The model with belongs_to is the child and links to one parent model.

Solution

1. Step 1: Understand the role of belongs_to

   The belongs_to association is used in the child model to link it to a single parent model.

2. Step 2: Identify the relationship direction

   This means the child model holds the foreign key and points to one parent, not many.

3. Final Answer:

   The model with belongs_to is the child and links to one parent model. -> Option D

4. Quick Check:

   belongs_to = child to one parent [OK]

Hint: Remember: belongs_to means child points to one parent [OK]

Common Mistakes:

• Confusing belongs_to with has_many
• Thinking belongs_to creates many-to-many
• Assuming belongs_to creates a new table

2. Which of the following is the correct syntax to declare a belongs_to association in a Rails model named Comment that belongs to a Post?

easy

A. belongs_to :post

B. belongs_to Post

C. has_many :post

D. belongs_to 'post'

Solution

1. Step 1: Recall Rails association syntax

   In Rails, associations use symbols for model names, so belongs_to :post is correct.

2. Step 2: Check other options for syntax errors

   belongs_to Post lacks symbol, has_many is wrong association, and quotes are not standard here.

3. Final Answer:

   belongs_to :post -> Option A

4. Quick Check:

   Use symbol for model name in belongs_to [OK]

Hint: Use symbol (colon) for model name in belongs_to [OK]

Common Mistakes:

• Using class name without colon
• Using has_many instead of belongs_to
• Using string instead of symbol

3. Given the following Rails models:

class Comment < ApplicationRecord
  belongs_to :post
end

class Post < ApplicationRecord
  has_many :comments
end

What will comment.post return if comment is a Comment instance linked to a Post with title "Hello"?

medium

A. The Post object with title "Hello"

B. An array of comments

C. A string "Hello"

D. nil

Solution

1. Step 1: Understand belongs_to usage

   The belongs_to :post means each comment has one post object accessible via comment.post.

2. Step 2: Identify what comment.post returns

   It returns the Post instance linked to that comment, which has the title "Hello".

3. Final Answer:

   The Post object with title "Hello" -> Option A

4. Quick Check:

   comment.post returns parent Post object [OK]

Hint: belongs_to returns the single parent object [OK]

Common Mistakes:

• Expecting an array instead of single object
• Expecting a string instead of object
• Assuming nil when association exists

4. You have this model code:

class Order < ApplicationRecord
  belongs_to :customer
end

When running order.customer, you get an error: undefined method 'customer'. What is the most likely cause?

medium

A. The Customer model is not defined.

B. The belongs_to should be has_many.

C. The customer_id column is missing in the orders table.

D. The association name should be plural: belongs_to :customers.

Solution

1. Step 1: Check database foreign key presence

   The belongs_to :customer requires a customer_id column in the orders table to work.

2. Step 2: Understand error cause

   If customer_id is missing, Rails cannot create the method customer, causing the error.

3. Final Answer:

   The customer_id column is missing in the orders table. -> Option C

4. Quick Check:

   Missing foreign key column causes undefined method error [OK]

Hint: Check foreign key column exists for belongs_to [OK]

Common Mistakes:

• Confusing belongs_to with has_many
• Assuming association name should be plural
• Ignoring missing database column

5. You want to create a Rails app where each Review belongs to a Product, and each product can have many reviews. You also want to ensure deleting a product deletes its reviews automatically. Which setup is correct?

hard

A. In Review model: belongs_to :product, dependent: :destroyIn Product model: has_many :reviews

B. In Review model: belongs_to :productIn Product model: has_many :reviews, dependent: :destroy

C. In Review model: has_many :productIn Product model: belongs_to :reviews, dependent: :destroy

D. In Review model: belongs_to :productIn Product model: has_many :reviews, dependent: :nullify

Solution

1. Step 1: Assign belongs_to in child model

   The Review model should have belongs_to :product to link each review to one product.

2. Step 2: Use has_many with dependent option in parent

   The Product model should have has_many :reviews, dependent: :destroy to delete reviews when product is deleted.

3. Final Answer:

   Review: belongs_to :product; Product: has_many :reviews, dependent: :destroy -> Option B

4. Quick Check:

   dependent: :destroy goes on has_many side [OK]

Hint: Put dependent: :destroy on has_many side to delete children [OK]

Common Mistakes:

• Putting dependent: :destroy on belongs_to side
• Mixing up has_many and belongs_to
• Using dependent: :nullify when deletion is wanted