Polymorphic associations in Ruby on Rails - Mini Project: Build & Apply

Polymorphic Associations in Rails

📖 Scenario: You are building a simple Rails app where users can comment on different types of content: articles and photos. Instead of creating separate comment tables for each content type, you will use polymorphic associations to let comments belong to either articles or photos.

🎯 Goal: Create a Rails setup where Comment can belong to either an Article or a Photo using polymorphic associations.

📋 What You'll Learn

Create Article and Photo models with a title attribute

Create a Comment model that belongs to a polymorphic commentable

Set up the associations in all models correctly

Demonstrate creating comments for both articles and photos

💡 Why This Matters

🌍 Real World

Polymorphic associations let you reuse one model (like Comment) for many different related models (like Article and Photo) without duplicating tables or code.

💼 Career

Understanding polymorphic associations is important for Rails developers to design flexible and maintainable database relationships in real applications.

1

Create Article and Photo models

Create two Rails models called Article and Photo. Each model should have a string attribute called title. Use Rails generators or write the model classes with title attribute.

Ruby on Rails

# Create Article and Photo models with a title attribute
# Your code here

Need a hint?

Use rails generate model Article title:string and rails generate model Photo title:string or define classes with title attribute.

2

Add Comment model with polymorphic association

Create a Rails model called Comment with a string attribute body. Add polymorphic references commentable to this model. This means Comment belongs to commentable polymorphically.

Ruby on Rails

class Article < ApplicationRecord
  # title:string attribute
end

class Photo < ApplicationRecord
  # title:string attribute
end

# Create Comment model with body:string and polymorphic commentable
# Your code here

Need a hint?

Use rails generate model Comment body:string commentable:references{polymorphic} or define the class with belongs_to :commentable, polymorphic: true.

3

Set up associations in Article and Photo models

In the Article and Photo models, add the association to comments using has_many :comments, as: :commentable. This sets up the polymorphic connection from the other side.

Ruby on Rails

class Article < ApplicationRecord
  # title:string attribute
  # Your code here
end

class Photo < ApplicationRecord
  # title:string attribute
  # Your code here
end

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  # body:string attribute
end

Need a hint?

Add has_many :comments, as: :commentable inside both Article and Photo classes.

4

Create comments for articles and photos

Write code to create an Article with title 'Rails Guide' and a Photo with title 'Sunset'. Then create a Comment with body 'Great article!' for the article, and a Comment with body 'Beautiful photo!' for the photo. Use the polymorphic association to link comments.

Ruby on Rails

class Article < ApplicationRecord
  has_many :comments, as: :commentable
  # title:string attribute
end

class Photo < ApplicationRecord
  has_many :comments, as: :commentable
  # title:string attribute
end

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  # body:string attribute
end

# Create Article and Photo instances
article = Article.create(title: 'Rails Guide')
photo = Photo.create(title: 'Sunset')

# Create comments for article and photo
# Your code here

Need a hint?

Create instances of Article and Photo with the given titles. Then create Comment instances linking to these using commentable: option.