dm_migrate_upgrade.rb

class Employee
  include DataMapper::Resource

  property :id, Serial
  property :name, String
  ...
end

class Manager < Employee
  property :position, String, :default => 'manager'
  ...
end

Manager.auto_migrate!
  the table will have following columns:
    id
    name

Manger.auto_upgrade!
  the table will have following columns:
    id
    name
    position