post.rb

class Post < ActiveRecord::Base
  attr_accessible :body, :price, :title

  validates_presence_of :title
  validates_length_of :title, minimum: 10
  validates_presence_of :body
  validates_numericality_of :price, greater_than: 0
end

post_wizard_controller.rb

class PostWizardController < ApplicationController
  include Wicked::Wizard
  steps '1_title', '2_body', '3_price'

  VALIDATIONS = {
    '1_title' => [:title],
    '2_body' => [:title, :body],
    '3_price' => [:title, :body, :price],
  }.freeze

  # Here PartialPost is a private property of this controller not shared in any
  # other part of the code so we make it a nested class.
  class PartialPost < Post
    attr_accessor :step
    attr_accessor :session

    def self.model_name
      Post.model_name
    end

    def save
      # Trigger ActiveRecord's validation
      rv = super(validate: false)

      if rv && self.step == VALIDATIONS.keys.last
        self.partial = false
        rv = super
        session.delete(:post_wizard) if rv
        rv
      else
        self.valid?
        (self.errors.messages.keys - VALIDATIONS[self.step]).each do |k|
          self.errors.messages.delete(k)
        end

        # Insert complicated custom validation manipulation here
        self.errors.empty?
      end
    end
  end

  def index
    post = Post.new
    post.save(validate: false)
    session[:post_wizard] ||= {}
    session[:post_wizard][:post_id] = post.id
    redirect_to wizard_path(steps.first)
  end

  def show
    if session[:post_wizard].try(:[], :post_id).present?
      @post = PartialPost.find(session[:post_wizard][:post_id])
    end

    render_wizard
  end

  def update
    @post = PartialPost.find(session[:post_wizard][:post_id])
    @post.attributes = params[:post]

    @post.step = step
    @post.session = session

    render_wizard(@post)
  end
end