#154 Polymorphic Association (revised)

May 19, 2012 | 11 minutes | Active Record, Routing

A polymorphic association allows a model to belong_to different types of other models. Here I show how to make a single comment model belong to articles, photos, and events.

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          terminal
        
rails g model comment content:text commentable_id:integer commentable_type
rake db:migrate
rails g controller comments index new

          migrations/create_comments.rb
        
def change
  create_table :comments do |t|
    t.text :content
    t.belongs_to :commentable, polymorphic: true
    t.timestamps
  end
  add_index :comments, [:commentable_id, :commentable_type]
end

          models/comment.rb
        
class Comment < ActiveRecord::Base
  attr_accessible :content
  belongs_to :commentable, polymorphic: true
end

          models/article.rb
        
class Article < ActiveRecord::Base
  attr_accessible :content, :name
  has_many :comments, as: :commentable
end

          routes.rb
        
resources :articles do
  resources :comments
end

resources :photos do
  resources :comments
end

resources :events do
  resources :comments
end

          comments_controller.rb
        
class CommentsController < ApplicationController
  before_filter :load_commentable
  
  def index
    @comments = @commentable.comments
  end

  def new
    @comment = @commentable.comments.new
  end

  def create
    @comment = @commentable.comments.new(params[:comment])
    if @comment.save
      redirect_to @commentable, notice: "Comment created."
    else
      render :new
    end
  end

private

  def load_commentable
    resource, id = request.path.split('/')[1, 2]
    @commentable = resource.singularize.classify.constantize.find(id)
  end

  # alternative option:
  # def load_commentable
  #   klass = [Article, Photo, Event].detect { |c| params["#{c.name.underscore}_id"] }
  #   @commentable = klass.find(params["#{klass.name.underscore}_id"])
  # end
end

          comments/index.html.erb
        
<%= link_to "New Comment", [:new, @commentable, :comment] %>

          comments/_form.html.erb
        
<%= form_for [@commentable, @comment] do |f| %>

          articles_controller.rb
        
def show
  @article = Article.find(params[:id])
  @commentable = @article
  @comments = @commentable.comments
  @comment = Comment.new
end