class SampleDocument
  include Mongoid::Document
  include Mongoid::RelationsDirtyTracking

  embeds_one  :foo
  has_many    :bars

  field :title, type: String
end

doc = SampleDocument.create
doc.foo = Foo.new(title: 'foo')
doc.bars << Bar.new(title: 'bar')
doc.title = 'New title'

doc.relations_changed?  # => true
doc.relation_changes    # => {"foo" => [nil, {"_id"=>"524c35ad1ac1c23084000040", "title" => "foo"}], "bars" => [nil, [{"_id"=>"524c35ad1ac1c23084000083"}]]}
doc.changed?            # => true
doc.changes             # => {"title" => [nil, "New title"], "foo" => [nil, {"_id"=>"524c35ad1ac1c23084000040", "title" => "foo"}], "bars" => [nil, [{"_id"=>"524c35ad1ac1c23084000083"}]]}

doc.save
doc.relations_changed?  # => false
doc.relation_changes    # => {}
doc.changed?            # => false
doc.changes             # => {}