signup.rb

class Signup < ActiveRecord::Base
  def self.at_step(n, &block)
    @@step_specific_code ||= {}
    @@step_specific_code[n] = block
  end
  
  at_step(1) do
    validates_presence_of :name, :email_address, :mobile_phone
    attr_accessible :name, :email_address, :mobile_phone
  end

  at_step(2) do
    validates_presence_of :username, :password
    attr_accessible :username, :password
  end
  
  def advance_step
    self.step += 1
    instance_eval @@step_specific_code[self.step] if @@step_specific_code[self.step]
    yield self
  end
end

SignupController.rb

class SignupController < ActionController::Base
  def step_two
    @signup = Signup.find(params[:id])
    @signup.advance_step do |signup|
      signup.attributes = params[:signup]
      signup.save!
    end
  end
end