class Parent
  include Mongoid::Document

  field :name
end

module Adoptable
  def self.included(base)
    base_name_symbol = base.name.pluralize.underscore
    base.field :cost, :type => Integer, :default => 0

    # ChildGroup
    class_name = base.name + 'Group'
    # :child_groups
    class_name_symbol = class_name.pluralize.underscore.to_sym
    klass = Object.const_set(class_name,Class.new)
    klass.class_eval do
      include Mongoid::Document
      field :quantity, :type => Integer, :default => 0
      embedded_in :parent, :inverse_of => class_name_symbol
      referenced_in :resource, :class_name => base.name, :inverse_of => class_name_symbol
    end

    # child has many child_groups, though this side of the association is never used
    base.references_many class_name_symbol

    Parent.embeds_many class_name_symbol
    # the following code creates the convenient accessor user.children
    Parent.class_eval do
      define_method(base_name_symbol) do
        send(class_name_symbol).map { |resource_group| resource_group.resource }
      end
    end
  end
end

class Child
  include Mongoid::Document
  include Adoptable

  field :name
end