bar_node.rb

class BarNode < Node

  field :bar_test

end

foo_node.rb

class FooNode < Node

  field :foo_test

end

node.rb

class Node

  include Mongoid::Document

  field :title

  belongs_to_related :parent, :class_name => "Node", :foreign_key => :node_id
  has_many_related :children, :class_name => "Node"

end