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Polymorphic Associations With Active Record

Model associations are one of the things that every new Ruby on Rails developer must understand in order to effectively use the framework. The concept behind these relationships is rather simple, and most people have little trouble grasping it once introduced.

In polymorphic associations, a model can fit into more than one model, on a single association. Like just about everything else in the Ruby on Rails world, there is a ‘Rails way’ to do polymorphic associations. This involves leveraging the support that is built in to Rails’ standard ORM, Active Record. You set up a polymorphic association in very much the same way that you would a typical has_many.

Here are the steps to implement the polymorphic association between these three models.


                        class Comment < ActiveRecord::Base
                        belongs_to :commentable, polymorphic: true #…


                        class Post < ActiveRecord::Base

                        has_many :comments, as: :commentable #…

                        class Profile < ActiveRecord::Base

                        has_many :comments, as: :commentable #…

                       class CreateComments < ActiveRecord::Migration

                       def change

                       create_table :comments do |t|

                       t.references :commentable, polymorphic: true #…




Here you can see that all we’re really doing is passing a couple of parameters to the :has_many and :belongs_to methods. The migration will generate two columns on the Comments table, ‘commentable_id’ and ‘commentable_type’. Behind the scenes, Active Record will wire up all of the helper methods that you’re used to using with normal associations. Now the comment model will work as a polymorphic association between Post and Profile model.

And there you have it, both posts and profiles can have their own comments. The obvious limitation here is that, since we’re only setting a single foreign key and type, a comment can only belong to one other object at a time. This may be the desired functionality for some use cases. Sometimes, however, you’ll want to be able to associate a single model with multiple other models of different types at the same time.


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