Reverse polymorphic associations in Rails

description.markdown

Polymorphic Associations reversed

It's pretty easy to do polymorphic associations in Rails: A Picture can belong to either a BlogPost or an Article. But what if you need the relationship the other way around? A Picture, a Text and a Video can belong to an Article, and that article can find all media by calling @article.media

This example shows how to create an ArticleElement join model that handles the polymorphic relationship. To add fields that are common to all polymorphic models, add fields to the join model.

reverse polymorphic.rb

class Article < ActiveRecord::Base
  has_many :article_elements
  has_many :pictures, :through => :article_elements, :source => :element, :source_type => 'Picture'
  has_many :videos, :through => :article_elements, :source => :element, :source_type => 'Video'
end

class Picture < ActiveRecord::Base
  has_one :article_element, :as =>:element
  has_one :article, :through => :article_elements
end

class Video < ActiveRecord::Base
  has_one :article_element, :as =>:element
  has_one :article, :through => :article_elements 
end

class ArticleElement < ActiveRecord::Base
  belongs_to :article
  belongs_to :element, :polymorphic => true
end

t = Article.new
t.article_elements  # []
p = Picture.new
t.article_elements.create(:element => p)
t.article_elements  # [<ArticleElement id: 1, article_id: 1, element_id: 1, element_type: "Picture", created_at: "2011-09-26 18:26:45", updated_at: "2011-09-26 18:26:45">]
t.pictures # [#<Picture id: 1, created_at: "2011-09-26 18:26:45", updated_at: "2011-09-26 18:26:45">]

Commenting here just to see if anyone knows: What would the db migrations look like for this scenario? has_many :through has always hurt my brain a little, so I'm having trouble writing the migration by hand. I've been looking at this StackOverflow, but so far I can't get it to work 😢

@hbarudin The polymorphic migration can be achieved as below :

create_table :article_elements do |t|
    ...
    t.timestamps null: false
    t.references :article, index: true
    t.references :element, polymorphic: true, index: true
  end

Polymorphic is a bad idea as there is no way to keep the database consistent due to no FK.

In Picture change it to: has_one :article, :through => :article_element

What is interesting is inventing some kind of a form helper for the Article (to change a Picture to Video etc.)

I have the same question as @kbighorse
The issue I have with the answer given by @ayrton is that the result would be an array of ArticleElements, not an array of Pictures and Videos.
I would like to provide an ActiveRecord helper on my Article that reads like Article.first.media that does not bring back a bunch of joins, but reaches through the join and assembles a list of what is on the other side of that polymorphic table, i.e. a mix of Pictures and Videos that are polymorphically related to a given Article
I suppose this could be done by asking for Article.first.pictures and joining that manually with Article.first.videos

This post explains why it is at least tricky if not impossible to traverse a polymorphic :through relationship in the "other" direction

One option would be

class Article < ActiveRecord::Base
  has_many :article_elements
  has_many :pictures, :through => :article_elements, :source => :element, :source_type => 'Picture'
  has_many :videos, :through => :article_elements, :source => :element, :source_type => 'Video'

  def media
    self.article_elements.collect{ |el| el.element }
  end
end