Polymorphic Associations

Rails has the concept of Polymorphic associations, which are associations that can be of different data types. For example, let’s say we have a Comment model. A comment in your app can be on a post or a page. Rather than creating a PostComment model and a PageComment model, you can set up your Comment model to have a polymorphic association to a Page or a Post.

Generally people call these associations “something-able”. In this case, it would be commentable.

Here’s a quick code sample:
class Comment < ActiveRecord::Base belongs_to :commentable, :polymorphic => true
end

class Page < ActiveRecord::Base has_many :comments, :as => :commentable
end

class Post < ActiveRecord::Base has_many :comments, :as => :commentable
end
The only other thing you need to know is the database structure. On the database, you need two columns. A [polymorphic-association-name]_type string column and [polymorphic-association-name]_id integer column. In this case, it would be “commentable_type” and “commen