Rails/ActiveRecord: belongs_to :polymorphic => true

This morning, I was looking for a way to have an ActiveRecord object “belong to” one of a handful of different object types. That’s when I came across the :polymorphic option of the belongs_to method.

From the description and the “Polymorphic” section of the docs it seemed like it would do what I need, so I set out to figure out how to use it. I was looking for an easy clear description of how it’s used, but several Google searches produced nothing. So I decided to make one myself.

We’ll start off with a little project used to track items in a “media library,” e.g. Books, Music, and Movies. Given the following models:

script/generate model Book author:string title:string summary:string isbn:string
script/generate model Album artist:string title:string track_listing:string
script/generate model Movie director:string title:string summary:string
script/generate model Artwork library_item_id:integer library_item_type:string

We want our Book, Album, and Movie objects to have one or more “Artwork” objects to represent covers or movie posters. First let’s tell the Artwork class it belongs to the other objects:

class Artwork < ActiveRecord::Base
  belongs_to :library_item, :polymorphic => true
end

Notice how the :library_item corresponds to the columns we defined: library_item_id and library_item_type. ActiveRecord is going to store the Book#id, Album#id, or Movie#id in the library_item_id. In the library_item_type column ActiveRecord will put "Book", "Album", or "Movie", as appropriate. For example, if you do this:

>> Artwork.new(:library_item => Book.create)
=> #<Artwork id: nil, library_item_id: 1, library_item_type: "Book", created_at: nil, updated_at: nil>

The library_item_id and library_item_type columns will have Book#id, in this case ’1′, and Book#class, which is Book, respectively.

Now here’s how the Book, Album, and Movie classes are tied to the “artwork”:

class Book < ActiveRecord::Base
  has_one :cover_image, :class_name => 'Artwork', :as => :library_item
end
class Album < ActiveRecord::Base
  has_one :cover_art, :class_name => 'Artwork', :as => :library_item
end
class Movie < ActiveRecord::Base
  has_one :poster_image, :class_name => 'Artwork', :as => :library_item
end

This gives us methods like Album#cover_art we can use to reference the Artwork object from the Album object.

So there you go, hopefully this proves useful…