A form writer to model pattern I use in Rails a lot.

list.rb

class List < ActiveRecord::Base
  attr_accessible :name, :items_to_add

  has_many :items

  # Allows me to easily build forms that do
  # complex things but still plug into 
  # mass-assignment.
  def items_to_add=(items)
    items.each{|i| add_item(i)}
  end
  
  # Don't expose the AR Api outside of the model
  # or to the form, rather, have an internal API
  # that form writers can use.
  def add_item(item)
  	# Also helps people maintain a consistent
    # way to add association, as opposed to
    # self.items << Item.new
    self.items.build(:name => item)
  end

end