model_wizard.rb

class ModelWizard
  attr_reader :object

  def initialize(object_or_class, session, params = nil, param_key = nil)
    @object_or_class = object_or_class
    @session = session
    @params = params
    @param_key = param_key || ActiveModel::Naming.param_key(object_or_class)
    @session_params = "#{@param_key}_params".to_sym
  end

  def start
    @session[@session_params] = {}
    set_object
    @object.current_step = 0
    self
  end

  def process
    @session[@session_params].deep_merge!(@params[@param_key]) if @params[@param_key]
    set_object
    @object.assign_attributes(@session[@session_params]) unless class?
    self
  end

  def save
    saved = false
    if @params[:back_button]
      @object.step_back
    elsif @object.current_step_valid?
      binding.pry
      if @object.last_step?
        if @object.all_steps_valid?
          saved = @object.save
          @session[@session_param] = nil
        end
      else
        @object.step_forward
      end
    end
    saved
  end

private

  def set_object
    @object = class? ? @object_or_class.new(@session[@session_params]): @object_or_class
  end

  def class?
    @object_or_class.is_a?(Class)
  end

end

multi_step.rb

module MultiStepModel
  attr_accessor :current_step

  def current_step
    @current_step.to_i
  end

  def current_step_valid?
    valid?
  end

  def all_steps_valid?
    (0...self.class.total_steps).all? do |step|
      @current_step = step
      current_step_valid?
    end
  end

  def step_forward
    @current_step = current_step + 1
  end

  def step_back
    @current_step = current_step - 1
  end

  def step?(step)
    @current_step.nil? || current_step + 1 == step
  end

  def last_step?
    step?(self.class.total_steps)
  end

  def first_step?
    step?(1)
  end

  def method_missing(method_name, *args, &block)
    if /^step(\d+)\?$/ =~ method_name
      step?($1.to_i)
    else
      super
    end
  end

end