Polymorphic associations in Rails

A model can associate with other models by way of a polymorphic association. This can be useful when the associated models are not of the same class but share a common interface, or when a model shares the same type of relationship with multiple other models.

For example, three models Post, Project, and Issue may have many comments.

Instead of creating a different comment model for each of these three models (PostComment, ProjectComment…) it would be much more straightforward to have a single Comment model and establish a relationship to a subject.

create_table :comments do |t| t.text :body t.references :subject, polymorhic: true 

The subject could be a Post, Project, Issue or any other model we wish to attach a comment to.

The polymorphic option, instructs the database to store the associated object's type. The above migration creates a :subject_type column.

The relationships are established in the models like so:

class Comment < ApplicationRecord belongs_to :subject, polymorphic: true end class Post < ApplicationRecord has_many :comments, as: subject end class Project < ApplicationRecord has_many :comments, as: subject end class Issue < ApplicationRecord has_many :comments, as: subject end