class Parent
  include Mongoid::Document
  field :foo, :type => String, :default => ->{ Moped::BSON::ObjectId.new.to_s }
end

class Child < Parent
  field :foo, :type => String, :default => ->{ Time.now.to_i.to_s }
end

Parent.new.foo
 => "5097db750bed3f4cf5000003" 
Child.new.foo
 => "5097db780bed3f4cf5000004" # expected something like "1352129386"