grouped_collection_select pre-select something

require 'ostruct'

include ActionView::Helpers::FormOptionsHelper

# fake ActiveRecord models

class Continent < OpenStruct
  #attr_accessor :id, :name, :countries
  def initialize(*args)
    super
    self.countries = []
  end
end

class Country < OpenStruct
  #attr_accessor :id, :name, :continent_id, :continent
  def initialize(*args)
    super
    self.cities = []
  end
end

class City < OpenStruct
  #attr_accessor :id, :name, :country_id, :country
end

# used to generate a sane combination of instances for our models
def example_data
  europe = Continent.new(:id => 1, :name => 'Europe')
  asia   = Continent.new(:id => 2, :name => 'Asia')

  germany = Country.new(:id => 3, :name => 'Germany', :continent_id => 1, :continent => europe)
  france  = Country.new(:id => 4, :name => 'France',  :continent_id => 1, :continent => europe)
  china   = Country.new(:id => 5, :name => 'China',   :continent_id => 2, :continent => asia)
  japan   = Country.new(:id => 6, :name => 'Japan',   :continent_id => 2, :continent => asia)

  berlin   = City.new(:id => 7,  :name => 'Berlin',   :country_id => 3, :country => germany)
  cologne  = City.new(:id => 8,  :name => 'Cologne',  :country_id => 3, :country => germany)
  paris    = City.new(:id => 9,  :name => 'Paris',    :country_id => 4, :country => france)
  brest    = City.new(:id => 10, :name => 'Brest',    :country_id => 4, :country => france)
  beijing  = City.new(:id => 11, :name => 'Beijing',  :country_id => 5, :country => china)
  shanghai = City.new(:id => 12, :name => 'Shanghai', :country_id => 5, :country => china)
  tokio    = City.new(:id => 13, :name => 'Tokio',    :country_id => 6, :country => japan)
  osaka    = City.new(:id => 14, :name => 'Osaka',    :country_id => 6, :country => japan)

  europe.countries = [germany, france]
  asia.countries   = [china, japan]

  germany.cities = [berlin, cologne]
  france.cities  = [paris, brest]
  china.cities   = [beijing, shanghai]
  japan.cities   = [tokio, osaka]

  {
    :continents => [europe, asia],
    :countries  => [germany, france, china, japan],
    :cities     => [berlin, cologne, paris, brest, beijing, shanghai, tokio, osaka]
   }
end

# setup is done, let's start with the fun part

e = example_data
@city = e[:cities][3]
puts grouped_collection_select(:city, :country_id, e[:continents], :countries, :name, :id, :name)

# outputs the following: 
# <select id="city_country_id" name="city[country_id]">
#   <optgroup label="Europe">
#     <option value="3">Germany</option>
#     <option value="4" selected="selected">France</option>
#   </optgroup>
#   <optgroup label="Asia">
#     <option value="5">China</option>
#     <option value="6">Japan</option>
#   </optgroup>
# </select>