Polymorphic associations in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a polymorphic association in Rails?

A polymorphic association lets a model belong to more than one other model using a single association. It uses a type and an ID to link to different models through one interface.

beginner

How do you declare a polymorphic belongs_to association in Rails?

Use belongs_to :attachable, polymorphic: true in the model that belongs to multiple other models.

beginner

What columns are required in the database for a polymorphic association?

You need two columns: one for the ID (e.g., <code>attachable_id</code>) and one for the type (e.g., <code>attachable_type</code>) to store the associated model's class name.

intermediate

How do you set up the other side of a polymorphic association?

Use has_many :attachments, as: :attachable in the models that can have attachments. The as: option tells Rails this is a polymorphic interface.

intermediate

Why use polymorphic associations instead of multiple separate associations?

Polymorphic associations reduce code duplication and database complexity by allowing one model to relate to many others without needing separate foreign keys or tables for each.

Which two columns are essential for a polymorphic association in Rails?

A attachable_key and attachable_name

B attachable_id and attachable_type

C attachable_key and attachable_type

D attachable_id and attachable_name

Rails uses attachable_id to store the associated record's ID and attachable_type to store the class name.

How do you declare a polymorphic association on the model that belongs to multiple models?

A belongs_to :attachable, polymorphic: true

B has_one :attachable, polymorphic: true

C has_many :attachable, polymorphic: true

D belongs_to :attachable

The model that belongs to multiple others uses belongs_to :attachable, polymorphic: true.

What does the as: option do in a has_many association?

A Declares a scope

B Specifies the foreign key name

C Sets the table name

D Defines the polymorphic interface name

The as: option tells Rails the name of the polymorphic interface to use.

Which of these is a benefit of polymorphic associations?

A They simplify code by using one association for many models

B They require more database tables

C They prevent models from sharing associations

D They only work with one model

Polymorphic associations let one model relate to many others through a single association, simplifying code.

If a Comment model belongs to either a Post or a Photo, what would the polymorphic association look like in Comment?

A has_many :commentable, polymorphic: true

B belongs_to :post_or_photo

C belongs_to :commentable, polymorphic: true

D has_one :commentable

The Comment model uses belongs_to :commentable, polymorphic: true to belong to either Post or Photo.

Explain how polymorphic associations work in Rails and why they are useful.

Think about how one model can connect to many different models using one link.

Describe the database setup needed to support a polymorphic association in Rails.

Focus on the columns that store the link to other models.

Practice

1. What is the main purpose of polymorphic associations in Rails?

easy

A. To allow a model to belong to more than one other model using a single association

B. To create a one-to-one relationship between two models

C. To enforce database constraints on foreign keys

D. To automatically generate views for multiple models

Solution

1. Step 1: Understand polymorphic association concept

   Polymorphic associations let one model belong to multiple other models through a single interface.

2. Step 2: Compare options with concept

   To allow a model to belong to more than one other model using a single association correctly describes this flexibility. Other options describe unrelated features.

3. Final Answer:

   To allow a model to belong to more than one other model using a single association -> Option A

4. Quick Check:

   Polymorphic association = flexible belongs_to [OK]

Hint: Polymorphic means one interface for many models [OK]

Common Mistakes:

• Thinking it creates one-to-one relationships
• Confusing with database foreign key constraints
• Assuming it auto-generates views

2. Which of the following is the correct syntax to declare a polymorphic belongs_to association in a Rails model?

easy

A. has_many :commentable, polymorphic: true

B. has_one :commentable, polymorphic: true

C. belongs_to :commentable, as: :polymorphic

D. belongs_to :commentable, polymorphic: true

Solution

1. Step 1: Recall belongs_to polymorphic syntax

   The correct syntax uses belongs_to :name, polymorphic: true in the model that belongs to multiple types.

2. Step 2: Match options with syntax

   belongs_to :commentable, polymorphic: true matches this exactly. Options A and D use wrong association types, and C uses incorrect keyword.

3. Final Answer:

   belongs_to :commentable, polymorphic: true -> Option D

4. Quick Check:

   belongs_to with polymorphic: true = belongs_to :commentable, polymorphic: true [OK]

Hint: belongs_to uses polymorphic: true, has_many uses as: :name [OK]

Common Mistakes:

• Using has_many instead of belongs_to for polymorphic
• Using 'as: :polymorphic' instead of 'polymorphic: true'
• Confusing has_one with belongs_to

3. Given these models:

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

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

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

What will post.comments.first.commentable_type return if the first comment belongs to a Post?

medium

A. "Photo"

B. "Comment"

C. "Post"

D. nil

Solution

1. Step 1: Understand polymorphic type storage

   The polymorphic association stores the class name of the associated object in commentable_type.

2. Step 2: Identify the associated model

   The comment belongs to a Post, so commentable_type will be "Post".

3. Final Answer:

   "Post" -> Option C

4. Quick Check:

   commentable_type stores class name = "Post" [OK]

Hint: commentable_type holds the class name of associated model [OK]

Common Mistakes:

• Confusing commentable_type with comment class
• Expecting the associated object instead of its type string
• Assuming nil if association exists

4. You have this model:

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

Which of these will cause an error when creating a Picture?

medium

A. Picture.create(imageable_type: nil, imageable_id: 2)

B. Picture.create(imageable_type: "Product", imageable_id: 2)

C. Picture.create(imageable_type: "User", imageable_id: 1)

D. Picture.create(imageable_type: "User", imageable_id: 3)

Solution

1. Step 1: Check required polymorphic keys

   Polymorphic belongs_to requires both imageable_type and imageable_id to be present for a valid association.

2. Step 2: Identify missing or nil values

   Picture.create(imageable_type: nil, imageable_id: 2) has imageable_type: nil, which breaks the association and causes an error.

3. Final Answer:

   Picture.create(imageable_type: nil, imageable_id: 2) -> Option A

4. Quick Check:

   Missing polymorphic type causes error [OK]

Hint: Both type and id must be present for polymorphic belongs_to [OK]

Common Mistakes:

• Assuming nil imageable_type is acceptable
• Ignoring missing type causes runtime error
• Confusing polymorphic keys with normal foreign keys

5. You want to add comments to both Articles and Events using polymorphic associations. Which setup correctly models this?

hard

A. class Comment < ApplicationRecord
belongs_to :article
belongs_to :event
end

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

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

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

C. class Comment < ApplicationRecord
has_many :articles
has_many :events
end

D. class Article < ApplicationRecord
belongs_to :comment
end

class Event < ApplicationRecord
belongs_to :comment
end

Solution

1. Step 1: Identify polymorphic association pattern

   Comments belong to a polymorphic commentable interface, allowing multiple models to share comments.

2. Step 2: Match correct model declarations

   class Comment < ApplicationRecord belongs_to :commentable, polymorphic: true end class Article < ApplicationRecord has_many :comments, as: :commentable end class Event < ApplicationRecord has_many :comments, as: :commentable end correctly uses belongs_to :commentable, polymorphic: true in Comment and has_many :comments, as: :commentable in Article and Event.

3. Final Answer:

   polymorphic belongs_to and has_many as: :commentable -> Option B

4. Quick Check:

   Polymorphic belongs_to + has_many as: = class Comment < ApplicationRecord belongs_to :commentable, polymorphic: true end class Article < ApplicationRecord has_many :comments, as: :commentable end class Event < ApplicationRecord has_many :comments, as: :commentable end [OK]

Hint: Use belongs_to polymorphic in child, has_many as: in parents [OK]

Common Mistakes:

• Trying to belongs_to both models separately
• Using has_many in Comment instead of belongs_to
• Assigning belongs_to comment in parent models