class Parent
  include Mongoid::Document
  include Mongoid::Timestamps

  embeds_many :child, class_name: 'Child', inverse_of: :parent, validate: false, cascade_callbacks: true
end

class Child
  include Mongoid::Document
  include Mongoid::Timestamps

  embedded_in :parent
end

raise unless child.some_attribute == 1
child.some_attribute = 2
child.save
new_child = Child.<get from db>
raise unless new_child.some_attribute == 2 # raises here