options_for_select method to accommodate sub-categories

category.rb

class Category < ActiveRecord::Base
  has_many :categories, :order => 'order_by asc'
  belongs_to :category

  scope :order, order("order_by asc")
  scope :main, where(:category_id => nil)

  def options_for_select( selected = nil )
    selected_attribute = ' selected="selected"' if self == selected
    options_output = "<option value='#{self.id}'#{selected_attribute}>#{self.title}</option>"
    self.categories.each do |category|
      options_output += category.options_for_select( selected )
    end
    options_output.html_safe
  end
end

category_view_prototype.html.erb

<select>
  <% Category.main.order.each do |category| %>
    <%= category.options_for_select Category.find(2) %>
  <% end %>
</select>