Rails - Jul 2006 - Problems with validates_lenght_of

I''m building a login system, and I want users to be able to update
their
password, first name and last name after signing up. So, when signing up 
password can''t be nil, and it must be between 6..20 characters long.
However,
on update, the user can choose to leave the password field empty (but fill in 
other stuff, like first name) and then I want the User model to not update 
the password. 

This is what I''ve tried so far (some code omitted):

class User < ActiveRecord::Base
  attr_accessor :password, :password_confirmation
  attr_accessible :login, :password, :password_confirmation,
                           :email, :first_name, :last_name
  validates_confirmation_of :password
  validates_presence_of :password, :password_confirmation, :login,
                                                        :email, :on =>
:create

  validates_length_of :password, :within => 6..20,
                                 :too_long => "must be shorter",
                                 :too_short => "must be longer", 
                                 :on => :create

  validates_length_of :password, :within => 6..20, 
                                 :too_long => "must be shorter", 
                                 :too_short => "must be longer", 
                                 :on => :update, :allow_nil => true

  before_update :before_create
  after_update :after_create

  def before_create
    if @password
      self.hashed_password = User.hash_password(@password)
    end
  end
  
  def after_create
    @password = nil
  end

...  

end

The idea is, that if the password supplied by the form is nil, the model
won''t
change hashed_password. However, the view gives me an error message about 
password being too short, when I leave the password field empty! Why is this, 
and how could I implement this idea in another way?

Thanks in advance,

Malte