gistfile1.rb

# project.rb

class Store
  include Mongoid::Document
end

class Project
  include Mongoid::Document
  field :name

  embeds_one :store, as: :project
end

# dropbox_store.rb

require 'project'

class DropboxStore < Store
  #include Mongoid::Document
  field :session
  field :path
  
  embedded_in :project, polymorphic: true
  
  validates_presence_of :path
  after_initialize :load_session
end