class Parent
    include Mongoid::Document

    embeds_many :things, :cascade_callbacks => true

    before_create :initialize_first_thing

    def initialize_first_thing
        self.things << Thing.new(:name => "first")
    end
end

class Thing
    include Mongoid::Document
    include Mongoid::Timestamps

    embedded_in :parent

    field :name, :type => String

    before_create lambda {puts "before_create"}
end