Polymorphic associations


In Ruby on Rails, associations are the glue that binds your models together. But what if a single model needs to connect with multiple other models, and not just one specific type? This is where polymorphic associations come in, offering a powerful and flexible way to structure your data relationships.

Understanding Polymorphism

Imagine a social media platform where users can post comments. These comments can be attached to various things: a status update, a photo, or even another comment. Defining a separate association for each possibility would be cumbersome and inflexible. Polymorphic associations come to the rescue!

The Rails Way: Two Columns, Many Possibilities

Polymorphism utilizes two additional columns in the associating table. One stores the ID of the related model, and the other stores the type of that model. This allows a single association to connect to any model that defines the expected behavior.

Real-World Analogies: Beyond Social Media

The power of polymorphic associations extends far beyond social media comments. Here are some real-life examples that illustrate their versatility:

  • E-commerce: A product can have many attachments, like images, videos, or specifications. A polymorphic association can link the Product model to these various attachment models.
  • Inventory Management: A Shipment could contain different types of items, like books, electronics, or clothes. Polymorphism allows the Shipment model to link to these diverse models through a single association.
  • Content Management System (CMS): A CMS might allow attaching files (documents, images) to various content types (pages, articles). Polymorphism elegantly handles this by connecting the File model to different models based on content type.

Benefits of Polymorphism: DRY Code and Flexibility

  • Reduced Code Duplication: By using a single polymorphic association, you avoid defining separate associations for each possible connection. This keeps your code DRY (Don’t Repeat Yourself) and easier to maintain.
  • Flexible Data Relationships: As your application grows, you can introduce new model types that can leverage the existing polymorphic association without code changes. This promotes adaptability and future-proofs your data model.

A code example

Here’s a simplified example demonstrating a polymorphic association in Rails:

# attachable.rb
class Attachable < ApplicationRecord
  belongs_to :attachable, polymorphic: true

# post.rb
class Post < ApplicationRecord
  has_many :attachments, as: :attachable

# image.rb
class Image < ApplicationRecord
  has_one :attachment, as: :attachable

In this example, the Attachable model acts as the bridge, storing the reference to the attached model (attachable_type) and its ID (attachable_id). The Post model can have many attachments (has_many), while an Image model can be attached to one model (has_one).


Polymorphic associations offer a powerful tool to model flexible data relationships in Ruby on Rails applications. By embracing polymorphism, you can create a more adaptable and maintainable codebase while keeping your data structure organized. So, the next time you encounter a situation where a single model needs to connect with various others, consider the power of polymorphism! If you have any doubts post them into the comment section.