blog.rb

class Blog
  include MongoMapper::Document

  # Attributes ::::::::::::::::::::::::::::::::::::::::::::::::::::::
  key :title, String
  key :category_id, ObjectId
  key :archive_after, Time

  # Assocations :::::::::::::::::::::::::::::::::::::::::::::::::::::
  belongs_to :category
  many :comments


  # Validations :::::::::::::::::::::::::::::::::::::::::::::::::::::

  # Callbacks :::::::::::::::::::::::::::::::::::::::::::::::::::::
  before_save :set_archive
  after_save :log_save

  def to_s
    puts "\n----------\n#{title}"
    comments.each do |c|
      c.to_s if c.comment_id.nil?
    end
  end

private
  def log_save
    puts "Blog created, will be archived on #{archive_after.strftime("%d %b %Y") unless archive_after.nil?}"
  end
  def set_archive
    puts "set archive"
    self.archive_after = Time.now + 30.days
  end
end

comment.rb

class Comment
  include MongoMapper::Document
  key :text, String

  many :comments
  key :comment_id, ObjectId
  belongs_to :comment

  key :blog_id, ObjectId
  belongs_to :blog

  def to_s(depth = 1)
    prefix = "\t"*depth
    puts "#{prefix}#{text}"
    comments.each do |c|
      c.to_s(depth+1)
    end
  end
end

nested_demo.rb

# Be sure to have mongod running
# Below is an attempt to show one way to have nested comments on a blog post
# I did not thoroughly examine whether it is bulletproof or performant. But it seemed to work :-)

$LOAD_PATH << File.expand_path(File.dirname(__FILE__) + "/../app/model")
require 'rubygems'
require 'mongo_mapper'
require 'blog'
require 'comment'

MongoMapper.connection = Mongo::Connection.new('127.0.0.1', 27017)
MongoMapper.database = 'demo'

col = Blog.all()
col.each {|c| c.destroy }
col = Comment.all()
col.each {|c| c.destroy }


blog = Blog.create(:title => "Greatest Idea Ever" )
c1 = Comment.create( :text => "Comment 1", :blog => blog)
c2 = Comment.create(:text => "Comment 2", :blog => blog)

c11 = Comment.create(:text => "Comment 1 - 1", :blog => blog, :comment => c1)
c12 = Comment.create(:text => "Comment 1 - 2", :blog => blog, :comment => c11)
blog.to_s