MongoMapper Polymorphism with Single Collection Inheritance (SCI)

polymorphic_playing.rb

class Commentable
  include MongoMapper::Document
  many :comments, :as => :commentable
end

class Article < Commentable
end

class Product < Commentable
end

class Comment
  include MongoMapper::Document
  key :text, String
  belongs_to :commentable, :polymorphic => true
end

article = Article.create
article.comments.create(:text => "Snazzy page!")
article.save
product = Product.create
product.comments.create(:text => "It broke after a month.  But it was a good month.")
product.comments.create(:text => "Flys like a dream...until it crashes.")
product.save
article.reload.comments.count # 1
product.reload.comments.count # 2
comment = product.comments.first
comment.to_mongo # =>
{
  "_id"  => BSON::ObjectId('...'),
  "text" => "It broke after a month.  But it was a good month.",
  "commentable_type" => "Product",
  "commentable_id"   => BSON::ObjectId('...')
}
comment.commentable.class # Product
comment.commentable.to_mongo # =>
{
  "_id"   => BSON::ObjectId('...'),
  "_type" => "Product"
}
new_product = Product.new
new_comment = Comment.create
new_comment.commentable = new_product
new_comment.save
new_comment.reload.commentable == new_product # true
new_product.reload.comments.first == new_comment # true