Writing a simple accessor ... or so I thought

rubyonrails-talk
#1

class User < ActiveRecord::Base
  attr_reader :login

  def login=(new_login)
    @login = new_login unless !new_record?
  end
end

Try this:
def login=(new_login)
  write_attribute( :login, new_login ) if new_record?
end

For more info take a look at the "Overwriting default accessors"
section of the ActiveRecord::Base documentation.
http://api.rubyonrails.com/classes/ActiveRecord/Base.html

Aaron