Output

$ ruby test.rb 
Parent (after_initialize): 
Parent (Outside object): #<Template:0x7f91a16d3078>

test.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'bundler'
Bundler.require(:default)
Mongoid.database = Mongo::Connection.new('localhost').db('mongoid_xml_test_2')

class Template
  include Mongoid::Document

  embeds_many :pages
end

class Page
  include Mongoid::Document

  embedded_in :template, :inverse_of => :pages

  after_initialize :do_processing

  def do_processing
    # Returns nil
    puts "Parent (after_initialize): #{self._parent}"
  end

end

t = Template.new
p = t.pages.new
# Returns the parent
puts "Parent (Outside object): #{p._parent}"