_typeがサブクラスを指すようにしたい。

a.rb

class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
end

class Product < ActiveRecord::Base
end

class PublicProduct < Product
  has_one :picture, as: :imageable
end

class PrivateProduct < Product
  has_one :picture, as: :imageable
end

PublicProduct.create.build_picture #=> { ..., imageable_type: "Product", ... }

migration.rb

class Migration < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :name
      t.boolean :published

      t.timestamps null: false
    end

    create_table :pictures do |t|
      t.string :url
      t.integer :imageable_id
      t.string :imageable_type

      t.timestamps null: false
    end
    add_index :pictures, [:imageable_id, :imageable_type]
  end
end

productsテーブルにtypeカラムはあえてつけてないんだけど付けても同じ。