Polymorphic associations in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does polymorphic association behave in Rails?

Given a polymorphic association where Comment belongs to commentable which can be either Post or Image, what will comment.commentable return if the comment belongs to a post?

Ruby on Rails

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

class Post < ApplicationRecord
  has_many :comments, as: :commentable
end

post = Post.create(title: "Hello")
comment = Comment.create(body: "Nice post!", commentable: post)

result = comment.commentable

A The Post object associated with the comment

B The Comment object itself

C An array of all comments for the post

D A string with the post's title

💡 Hint

Think about what belongs_to :commentable means in Rails.

The commentable method returns the actual object the comment belongs to, which is the Post instance in this case.

📝 Syntax

intermediate

Identify the correct polymorphic association syntax

Which option correctly defines a polymorphic association in Rails?

A

class Picture < ApplicationRecord
  belongs_to :imageable
end

B

class Picture < ApplicationRecord
  has_many :imageable, polymorphic: true
end

C

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
end

D

class Picture < ApplicationRecord
  has_one :imageable, polymorphic: true
end

💡 Hint

Remember that the model holding the foreign key uses belongs_to with polymorphic: true.

The model that stores the polymorphic foreign keys uses belongs_to :association_name, polymorphic: true. Other models use has_many or has_one without polymorphic.

🔧 Debug

advanced

Why does this polymorphic association raise an error?

Given the following code, why does comment.commentable raise ActiveRecord::RecordNotFound?

Ruby on Rails

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

comment = Comment.create(body: "Test", commentable_type: "Post", commentable_id: 9999)
comment.commentable

A Because belongs_to requires inverse_of option for polymorphic

B Because there is no Post record with id 9999 in the database

C Because commentable_type should be lowercase 'post'

D Because the polymorphic association is missing the :dependent option

💡 Hint

Check if the associated record exists in the database.

The error occurs because Rails tries to find a Post with id 9999, which does not exist, causing ActiveRecord::RecordNotFound.

❓ state_output

advanced

What is the count of comments for a polymorphic association?

Given a Post with 3 comments and an Image with 2 comments, what is the output of Comment.where(commentable_type: 'Post').count?

Ruby on Rails

post = Post.create(title: "Post")
image = Image.create(name: "Image")
3.times { Comment.create(body: "Post comment", commentable: post) }
2.times { Comment.create(body: "Image comment", commentable: image) }

result = Comment.where(commentable_type: 'Post').count

A 3

B 5

C 2

D 0

💡 Hint

Filter comments by the commentable_type column.

The query counts only comments where commentable_type is 'Post', which are 3.

🧠 Conceptual

expert

Why use polymorphic associations in Rails?

Which is the best explanation for why polymorphic associations are useful in Rails?

A They replace the need for join tables in many-to-many relationships

B They enforce strict foreign key constraints between models

C They automatically generate database indexes for faster queries

D They allow a model to belong to more than one other model on a single association

💡 Hint

Think about how one model can be linked to different models using the same association.

Polymorphic associations let a model belong to multiple other models through one association, making code simpler and more flexible.