class Parent
  include Mongoid::Document
  include Mongoid::Timestamps
  embeds_many :kids, cascade_callbacks: true
end

class Kid
  include Mongoid::Document
  embedded_in :parent, touch: true
  field :name, type: String
end

p = Parent.new
p.kids << (k = Kid.new)
p.save

k.name = "test"
k.save