gistfile1.rb

class Page
  include MongoMapper::Document
end

class ContentPage < Page
  many :content_blocks #tried :as => :page as well
end

class ContentBlock
  include MongoMapper::EmbeddedDocument
  belongs_to :content_page
  # tried  belongs_to :page
  # and    belongs_to :page, :polymorphic => true

  # embedded_in :page didn't make any difference
  key :text, String
end

p = Page.find('4e42a6c0cb0f3d2bdc00021e') # is a Page
c = Page.find('4e42da66cb0f3d51f6000045') # is a ContentPage
c.content_blocks
# behaves as expected in mmconsole: [#<ContentBlock _id: BSON::ObjectId('4e8afbe0cb0f3d7846000012'), text: "foo">]
# but not in rails dev console/app: []
# or production console/app: [{"_id"=>BSON::ObjectId('4e78f6efcb0f3d56e80000a6'), "text"=>"foo"}]