MonkeyForms::Form is an ActiveModel-compliant interface between your controllers and models (or whatever you are saving the data to).

MonkeyForms supports multi-page wizards and validation groups.

class OrderForm
  include MonkeyForms::Form
  form_name 'cart'
  form_attributes :name, :address, :city, :state, :zip 

  validates :name, :presence => true

  # Save the data however you want.
  def save
    address = Address.new(:street => address, :city => city, :state => state, :zip => zip)
    OrderService.place_order(:the_name => name, :address => address)
  end
end


In your controller:
  def new
    @cart = OrderForm.new
  end
  def create
    @cart = OrderForm.new(:form => params[:cart])
    if @cart.valid?
      @cart.save
      redirect_to "/thanks"
    else
      render :action => 'new'
    end
  end
end

In your view:
  = form_for @cart do |f|
    = f.text_field :name
    = f.text_field :address
    = f.text_field :city
    = # etc
    = f.submit

A more complex multi-page order process. State is remembered in a cookie; the form params are merged into the cookie's state on each request.

class OrderForm
  include MonkeyForms::Form

  # Declares a few attributes on the form.
  form_attributes :name, :email, :city, :state, :line_items
  custom_attributes :user_id
  form_name :cart

  # This form serializes the submit into a gzip'd cookie with a name
  # of 'order_cookie'.
  set_form_storage(
    MonkeyForms::Serializers::GzipCookie.new(
      :name => 'order_cookie',
      :domain => 'test.domain.com',
      :secure => true,
      :httponly => true))

  after_initialize :set_default_state

  # We must submit an email address for the form to validate.
  validates :email, :presence => true

  validation_group :cart do
    # Scope some of the validation checks
    validates :name, :presence => true
  end

  validation_group :address do
    validates :city,  :presence => true
    validates :state, :presence => true
  end

  # This is a method that uses some form attributes.
  def person
    "#{ name } <#{ email }>"
  end

  private

  def set_default_state
    if state.blank?
      self.state = "WA"
    end
  end
end


class Controller
  before_filter :load_cart
  # Name / Email
  def page1
    if request.post?
      if group.valid?(:name)
        redirect_to "/page2"
      end
    end
  end

  # Address
  def page2
    if request.post?
      if group.valid?(:address)
        # whatever..
      end
    end
  end

  private

  def load_cart
    @cart = OrderForm::Form.new(:form => params[:cart]
  end
end

The validation_group code is modified from https://github.com/adzap/grouped_validations

This is pretty similar to the Presenter Pattern as described by Jay Fields, by the way. http://blog.jayfields.com/2007/03/rails-presenter-pattern.html

There is a sample sinatra application in test/sinatra. Run with: cd test/sinatra rackup config.ru

??? WHY ???

Moving the form logic to a separate class has a ton of advantages:

• Keeps the controller really simple.
  
• Makes it easier to test. You can write tests directly against the form handling class.
• Classes should do one thing.
• You can have complex validations.
• Your ActiveRecord models can probably become simpler.
• Since the form handling logic is encapsulated into one class, you can use inheritance, modules, etc.
• You want to move away from ActiveRecord? It's no problem -- just change how the form values are saved in the #save method.