1-vapor.rb

# VAlidate Part Of Resource
module Vapor
  extend ActiveSupport::Concern

  included do
    class_attribute :target_name, :attribute_names
    include ActiveModel::Validations
    include ActiveModel::Conversion
    include ActiveModel::Naming
  end

  def initialize(target, params = {})
    instance_variable_set("@#{target_name}", target)
    attribute_names.each do |attr|
      instance_variable_set("@#{attr}", params.try(:[], attr) || "")
    end
  end

  module InstanceMethods
    # for form helpers
    def persisted?
      false
    end
  end

  module ClassMethods
    def vaporize(target_name, *attributes)
      self.target_name = target_name
      self.attribute_names = attributes
      attr_accessor(*attributes)
      attr_accessor :principal
    end
  end
end

2-password_changer.rb

# Virtual Model for form helpers
class PasswordChanger
  include Vapor
  vaporize :user, :current_password, :new_password, :new_password_confirmation

  validate :check_user_credentials
  validates_length_of :new_password, :in => 8..256
  validates_confirmation_of :new_password

  def change_password!
    if valid?
      user.set_password(new_password)
    end
  end

  private
  def check_user_credentials
    unless user.verify_password(current_password)
      errors.add(:current_password, :incorrect)
    end
  end
end

3-users_controller.rb

class UsersController <ApplicationController
  
  # ...

  def edit_password
    @password_changer = PasswordChanger.new(@user, params[:password_changer])
    if request.post? && @password_changer.change_password!
      flash.notice = "Your password has been changed"
      redirect_to :controller => :dashboard, :action => :index
    end
  end

  # ...

end

4-edit_password.html.haml

= semantic_form_for @password_changer, :url => edit_password_path do |f|
  = f.input :current_password
  = f.input :new_password
  = f.input :new_password_confirmation
  = f.submit