Polymorphic associations in Ruby on Rails - Step-by-Step Execution

Concept Flow - Polymorphic associations

Define polymorphic model

↓

Add polymorphic reference to other models

↓

Create records linking to different models

↓

Access associated records via polymorphic interface

↓

Use polymorphic methods to get related objects

Shows how a model can belong to multiple other models through a single interface using polymorphic references.

Execution Sample

Ruby on Rails

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

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

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

Defines a Comment model that can belong to different models like Post or Image using polymorphic association.

Execution Table

Step	Action	Evaluation	Result
1	Create Post record	Post.create(title: 'Hello')	Post with id=1 created
2	Create Comment linked to Post	Comment.create(body: 'Nice post', commentable: Post#1)	Comment with id=1 linked to Post#1
3	Create Image record	Image.create(name: 'Pic')	Image with id=1 created
4	Create Comment linked to Image	Comment.create(body: 'Great image', commentable: Image#1)	Comment with id=2 linked to Image#1
5	Access commentable of Comment#1	Comment.find(1).commentable	Returns Post#1
6	Access commentable of Comment#2	Comment.find(2).commentable	Returns Image#1
7	Access comments of Post#1	Post.find(1).comments	Returns Comment#1
8	Access comments of Image#1	Image.find(1).comments	Returns Comment#2
9	Exit	No more actions	Polymorphic links established and accessible

💡 All polymorphic associations created and verified

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Post#1	nil	Post(id=1, title='Hello')	Post(id=1, title='Hello')	Post(id=1, title='Hello')	Post(id=1, title='Hello')	Post(id=1, title='Hello')	Post(id=1, title='Hello')
Image#1	nil	nil	nil	Image(id=1, name='Pic')	Image(id=1, name='Pic')	Image(id=1, name='Pic')	Image(id=1, name='Pic')
Comment#1	nil	nil	Comment(id=1, body='Nice post', commentable=Post#1)	Comment(id=1, body='Nice post', commentable=Post#1)	Comment(id=1, body='Nice post', commentable=Post#1)	Comment(id=1, body='Nice post', commentable=Post#1)	Comment(id=1, body='Nice post', commentable=Post#1)
Comment#2	nil	nil	nil	nil	Comment(id=2, body='Great image', commentable=Image#1)	Comment(id=2, body='Great image', commentable=Image#1)	Comment(id=2, body='Great image', commentable=Image#1)

Key Moments - 3 Insights

Why does Comment belong_to :commentable instead of a specific model like Post?

Because :commentable is polymorphic, Comment can link to different models (Post, Image, etc.) using the same interface, as shown in steps 2 and 4 where comments link to different models.

How does Rails know which model a comment belongs to?

Rails uses two columns: commentable_type (model name) and commentable_id (record id). This is why in step 5 accessing commentable returns the correct model instance.

Can we access comments from both Post and Image models?

Yes, because both models declare has_many :comments, as: :commentable, so steps 7 and 8 show accessing comments from different models.

Visual Quiz - 3 Questions

Look at the execution table, what does Comment#1's commentable refer to at step 5?

A Image#1

B Post#1

C Comment#2

D nil

💡 Hint

Check step 5 in the execution_table where Comment#1's commentable is accessed.

At which step is a Comment linked to an Image created?

A Step 4

B Step 6

C Step 2

D Step 8

💡 Hint

Look for the step where Comment.create links to Image#1 in the execution_table.

If we add a new model Video with comments, which part of the flow changes?

A Define polymorphic model

B Create records linking to different models

C Add polymorphic reference to Video

D Access associated records

💡 Hint

Adding a new model requires adding polymorphic references to that model, as shown in the concept_flow step 2.

Concept Snapshot

Polymorphic associations let a model belong to multiple other models through one interface.
Use belongs_to :name, polymorphic: true in the polymorphic model.
Other models use has_many :name, as: :name to link.
Rails stores type and id to track the association.
Access related objects via the polymorphic interface transparently.

Full Transcript

Polymorphic associations in Rails allow one model to belong to more than one other model using a single association. For example, a Comment can belong to a Post or an Image. This is done by adding belongs_to :commentable, polymorphic: true in the Comment model. The Post and Image models then declare has_many :comments, as: :commentable. When creating comments, Rails stores the type of the associated model and its id. This lets you access the associated object easily, like comment.commentable returning either a Post or Image instance. This approach avoids creating separate associations for each model and keeps code flexible and clean.