gistfile1.rb

%w(rubygems mongoid rspec).each do |gem| 
  require gem
end
Mongoid.configure do |config|
  name = "dirty_track_test"
  host = "localhost"
  config.master = Mongo::Connection.new.db(name)
  config.persist_in_safe_mode = false
end

RSpec.configure do |config|
  config.before(:each) do
    Mongoid.master.collections.reject { |c| c.name == 'system.indexes'}.each(&:drop)
  end
end

class Person
  include Mongoid::Document
  
  field :name
  
  embeds_one :address
  embeds_many :phones
  references_many :projects
  # dirty tracking of embedded documents
  attr_writer :embedded_changes, :referenced_changes
  
  def referenced_changes
    puts "SELF: #{(self.methods - Object.methods).sort.to_yaml}"
    puts "CHILDREN: #{((self.associations)).inspect}"
    
    @referenced_changes ||= begin
      self.associations.keys.each do |assoc_name|
        self.send(assoc_name).inject({}) do |memo, obj|
          memo.merge(obj.each{|c| c.send(:changes) if c.send(:changed?)})
        end
      end
    end
  end
  
  def embedded_changes
    @embedded_changes ||= begin
      self._children.inject({}) do |memo, child|
        memo.merge(child.changes) if child.changed?
      end
    end
  end

  def changes
    original_value = super
    if original_value.blank?
      embedded_changes   # need to merge these two hashes
      referenced_changes # but make sure to detect nil values
    else
      original_value.merge(embedded_changes).merge(referenced_changes)
    end
  end

  def changed?
    super || self._children.any?(&:changed?) # what about changes to referenced docs?
  end
  
end

class Address
  include Mongoid::Document
  field :city
  embedded_in :person, :inverse_of => :address
end

class Phone
  include Mongoid::Document
  field :number, :type => Integer
  embedded_in :person, :inverse_of => :phones
end

class Project
  include Mongoid::Document
  field :title
  referenced_in :person, :inverse_of => :projects
end

describe Person do
  context "parent document" do
    it "should detect the changes" do
      @person = Person.new(:name => "Sachin")
      @person.save!
      @person.name = "Sagar"
      @person.changed.should be_true
    end
  end
  context "parent document with embedded one document" do
    it "should detect the changes of the embedded one address document when the address city is changed" do
      @person = Person.new(:name => "Sachin")
      @person.create_address(:city => "Kathmandu")
      
      @person.changed?.should be_false
      @person.address.city = "Lalitpur"
      
      @person.changed?.should be_true
      @person.changes["city"].should include("Lalitpur")
    end
  end
  context "parent document with embedded many document" do
    it "should detect the changes of the embedded many phones document when the phone number is changed" do
      @person = Person.new(:name => "Sachin")
      @person.phones.create!(:number => 980)
      
      @person.phones.first.number.should == 980
      @person.changed?.should be_false
      @person.phones.first.number = 400
      
      @person.changed?.should be_true
      @person.changes["number"].should include(400)
    end
  end
  context "parent document with references many documents" do
    it "should detect the changes of the referenced many project document when the project title is changed" do
      @person = Person.create!(:title => "Sachin")
      @project = Project.new(:name => "Gliding")
      @person.projects << @project
      @person.changes
      @person.changed?.should be_true
      # @person.changes["title"].should include("Gliding")
    end
  end
end

Yes, I know dirty_associations is for activerecord. If you look at the code for dirty_associations, you can get some ideas on how to do something similar for mongoid. But I think you'll see that it won't be easy! Also, dirty_associations doesn't record changes to an associated record, only the addition or removal of an associated record.