signup.rb

class Signup < ActiveRecord::Base
  validates_presence_of :name, :email_address, :mobile_phone, :username, :password

  # knocked off variation of something toolmantim posted years ago
  def valid_for_attributes?(*attrs)
    return true if valid?
    attrs.collect! {|attr| attr.to_s }
    !errors.any? {|attr, error| attrs.include?(attr) }
  end

end

signups_controller.rb

# Multi step form which validates only submitted attributes per step
# but catches all on the final step to make sure model is valid.
# Doesn't prevent attribute hacking for each step but will ensure
# final record is valid.

class Signups < ApplicationController
  FINAL_STEP = 2

  def create
    @signup = Signup.new(params[:signup]
    if steps_complete?
      return redirect_to(elsewhere_path) if @signup.save
    elsif step_valid?
      @step += 1
    else
      # not complete or valid renders same step again with errors
    end
    # render partial for step in template
    render :action => 'new' 
  end

  private

  def current_step
    # also upper limit check for homework
    @step ||= (params[:step] || 1).to_i
  end

  def steps_complete?
    current_step == FINAL_STEP
  end

  def step_valid?
    @signup.valid_for_attributes(params[:signup].keys)
  end
end