class Parent
  include Mongoid::Document
  
  field :surname, :type => String
  
  has_many :children, :dependent => :destroy
end

class Child
  include Mongoid::Document
  
  belongs_to :parent
  
  field :surname, :type => String, :default => lambda { parent.surname if parent }
end

parent = Parent.create!(:surname => "Bond")

p Child.new(:parent => parent).surname # "Bond" :D
p parent.children.build.surname # nil :(