Implementing Abstract Base Model Class, the Rails Way™

I have a Book and Download model that share many attributes, so my goal is to inherit the common attributes from a DownloadableResource model.
Had a look at STI, but I went the abstract base model class way instead:

• models:

  class DownloadableResource < ActiveRecord::Base
    self.abstract_class = true
  
    attr_accessible :title, :url, :description, :active, :position
    validates :title, :url, :description, presence: true
    scope :active, where(active: true).order(:position)
  end
  
  class Book < DownloadableResource
    attr_accessible :cover_url, :authors
    validates :cover_url, :authors, presence: true
  end
  
  class Download < DownloadableResource
    attr_accessible :icon_url
    validates :icon_url, presence: true
  end
  

• migrations:

  class CreateDownloadableResources < ActiveRecord::Migration
    def change
      create_table :downloadable_resources do |t|
        t.string    :title
        t.string    :url
        t.text      :description
        t.boolean   :active,      default: false
        t.integer   :position
        t.timestamps
      end
    end
  end
  
  class CreateBooks < ActiveRecord::Migration
    def change
      create_table :books do |t|
        t.string :cover_url
        t.string :authors
        t.timestamps
      end
    end
  end
  
  class CreateDownloads < ActiveRecord::Migration
    def change
      create_table :downloads do |t|
        t.string :icon_url
        t.timestamps
      end
    end
  end
  

After migration, when I create a new Book the result is far from expected:

> Book.new
=> #<Book id: nil, cover_url: nil, authors: nil, created