gistfile1.rb

require 'rubygems'
require 'datamapper'

DataMapper.setup(:default, 'sqlite::memory:')

class Test
  include DataMapper::Resource

  property :id, Serial
  property :prop, String

  def set_prop(value)
    prop = value
  end
end

t = Test.new
t.set_prop 'a'
puts t.prop.nil?