irb(main):001:0> u=User.create(:username=>"admin")
=> #<User username="admin" password_salt=nil id=3 password_hash=nil>
irb(main):002:0> u.password = "admin"
=> "admin"
irb(main):003:0> u
=> #<User username="admin" password_salt="oWzS4nVz" id=3 password_hash="6524521271fddd58611eb5aa8b32899652864a15517d84cd6d878339494df109">
irb(main):004:0> User.first
=> #<User username="admin" password_salt=nil id=3 password_hash=nil>
## class User
require 'digest/sha2'
class User
  include DataMapper::Resource
  validates_is_unique   :username
  property :id, Integer, :serial => true
  property :username, String
  property :password_salt, String
  property :password_hash, String
  def password=(pass)
    salt = [Array.new(6){rand(256).chr}.join].pack("m").chomp
    self.password_salt, self.password_hash = salt, Digest::SHA256.hexdigest(pass+salt)
  end
  def self.authenticate( username, password )
    user = User.first :conditions => ['username = ?', username]
    if user.blank? ||
      Digest::SHA256.hexdigest( password + user.password_salt ) != user.password_hash
      raise "Username or password invalid"
    end
    user
  end
end