activemodelsimpleform.rb

class Form
  include ActiveModel::Validations
  extend ActiveModel::Naming

  def self.wraps(model)
    @wraps = model
  end

  def self.model_name
    ActiveModel::Name.new(@wraps.to_s.classify.constantize)
  end

  def initialize(model, attributes = {})
    @model = model
    attributes.each do |name, value|
      send("#{name}=", value)
    end
  end

  def to_key
    @model.to_key
  end

  def attributes
    Hash.new.tap do |atts|
      instance_variables.each do |ivar|
        next if [:@errors, :@model].include?(ivar)

        att = ivar[1..-1].to_sym
        atts[att] = send(att)
      end
    end
  end
end

edit_person_form.rb

class EditPersonForm < Form
  wraps :person

  attr_accessor :first_name, :last_name

  validates_presence_of :first_name, :last_name
end

people_controller.rb

def update
  @form = EditPersonForm.new(person, params[:person])
  if @form.valid?
     person.update_attributes(@form.attributes)
  else
     render :edit
  end
end

def person
  Person.find(params[:id])
end