gistfile1.rb

class User < ActiveRecord::Base
  attr_accessor :role
  has_many :user_roles, :dependent => :destroy
  has_many :roles, :through => :user_roles
  after_save :set_role

  def set_role
    if @role     # NOTE (A): @role is nil. I want it to be params[:user][:role].
      # NOTE (B): do stuff here, mostly:
      self.roles << @role
    end
  end
end

...

# in users_controller.rb:

def update
  @user = Uesr.find_by_login(params[:id])
  ...
  # at this point, params[:user][:role] has a value, the one given in the form!
  @user.update_attributes(params[:user])
end