Example decorator that could be used with GOV.UK API responses to extract tags by type

tag_type_decorator.rb

require 'active_support/inflector'

class TagTypeDecorator
  def self.has_tag_types(*types)
    types.each do |sym|
      define_method sym do
        taggable.tags.select { |t| t.tag_type == sym.to_s.singularize.capitalize }
      end
    end
  end

  has_tag_types :sections, :propositions, :audiences
  attr_accessor :taggable

  def initialize(taggable)
    raise "Isn't taggable" unless taggable.respond_to?(:tags)

    @taggable = taggable
  end

  def method_missing(method, *args)
    args.empty? ? taggable.send(method) : taggable.send(method, *args)
  end
end

require 'minitest/autorun'

class DecoratorTest < MiniTest::Unit::TestCase
  class Tag
    attr_accessor :tag_id, :tag_type
    def initialize(attrs = {})
      attrs.each { |k, v| __send__("#{k}=", v) }
    end
  end

  class SampleArtefact
    attr_accessor :stored_title

    def title
      @stored_title ||= "Bizarre"
      @stored_title
    end

    def title=(value)
      @stored_title = value
    end

    def tags
      [
        Tag.new(tag_id: 'driving/cars', tag_type: 'Section'),
        Tag.new(tag_id: 'driving/bicycles', tag_type: 'Section'),
        Tag.new(tag_id: 'policemen', tag_type: 'Audience'),
        Tag.new(tag_id: 'business', tag_type: 'Proposition')
      ]
    end
  end

  def setup
    @artefact = SampleArtefact.new
    @decorator = TagTypeDecorator.new(@artefact)
  end

  def test_it_gives_access_to_sections
    assert_equal ['driving/cars', 'driving/bicycles'], @decorator.sections.collect(&:tag_id)
  end

  def test_it_gives_access_to_propositions
    assert_equal ['business'], @decorator.propositions.collect(&:tag_id)
  end

  def test_it_gives_access_to_audiences
    assert_equal ['policemen'], @decorator.audiences.collect(&:tag_id)
  end

  def test_it_should_retain_access_to_raw_tags
    assert_equal 'driving/cars', @decorator.tags.first.tag_id
  end

  def test_it_should_retain_access_to_artefact_title
    assert_equal 'Bizarre', @decorator.title
  end

  def test_it_should_retain_access_to_artefact_title_setter
    @decorator.title= "Normal"
    assert_equal 'Normal', @decorator.title
  end
end