bats_and_horses.rb

require 'rubygems'
require 'dm-core'
require 'dm-migrations'

class User
  include DataMapper::Resource


  property :id, Serial
  property :login, String
  attr_accessor :foo

  before :save, :bar

  def bar
    self.login = @foo || "horse"
  end
end

DataMapper.setup :default, "sqlite3:data.sqlite"
DataMapper.auto_upgrade!

z = User.create
# login is "horse"

z.foo = "bat"
z.save

p z.reload.login
# i'd like login to be "bat", but it's "horse" :(