When do we nest class?

class Sidebar < ActiveRecord::Base
  serialize :config

  class Field
    attr_accessor :key
    attr_accessor :options
    attr_accessor :default
    include ApplicationHelper
    include ActionView::Helpers::TagHelper
    include ActionView::Helpers::FormTagHelper
    include ActionView::Helpers::FormOptionsHelper

    def initialize(key, default, options = { })
      @key, @default, @options = key.to_s, default, options
    end

    class SelectField < self
      def input_html(sidebar)
        select_tag(input_name(sidebar),
                   options_for_select(options[:choices],
sidebar.config[key]),
                   options)
      end
    end

    class TextAreaField < self
      def input_html(sidebar)
        html_options = { "rows" => "10", "cols" => "30", "style" =>
"width:255px"}.update(options.stringify_keys)
        text_area_tag(input_name(sidebar), h(sidebar.config[key]),
html_options)
      end
    end

end