dm_photo_and_photo_set_taggings.rb

#!/usr/bin/env ruby
#
# encoding: utf-8

require 'dm-core'
require 'dm-migrations'

DataMapper::Logger.new $stdout, :debug
DataMapper.setup :default, "sqlite::memory:"

class Photo
  include DataMapper::Resource

  property :id,      Serial
  property :caption, String

  has n, :photo_tags
  has n, :tags, :through => :photo_tags
end

class PhotoTag
  include DataMapper::Resource

  belongs_to :photo, :key => true
  belongs_to :tag,   :key => true
end

class PhotoSet
  include DataMapper::Resource

  property :id, Serial

  has n, :photo_set_tags
  has n, :tags, :through => :photo_tags

  has n, :photos, :through => :tags
end

class PhotoSetTag
  include DataMapper::Resource

  belongs_to :photo_set, :key => true
  belongs_to :tag,       :key => true
end

class Tag
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :unique => true

  has n, :photo_tags
  has n, :photos, :through => :photo_tags

  has n, :photo_set_tags
  has n, :photos, :through => :photo_tags
end

DataMapper.finalize
DataMapper.auto_migrate!

red_tag   = Tag.create(:name => 'red')
green_tag = Tag.create(:name => 'green')
blue_tag  = Tag.create(:name => 'blue')

Photo.create(:caption => "Tagged with red",            :tags => [ red_tag ])
Photo.create(:caption => "Tagged with green",          :tags => [ green_tag ])
Photo.create(:caption => "Tagged with green and blue", :tags => [ green_tag, blue_tag ])

photo_set_red            = PhotoSet.create(:tags => [ red_tag ])
photo_set_green_and_blue = PhotoSet.create(:tags => [ green_tag, blue_tag ])

puts photo_set_red.photos.inspect
puts photo_set_green_and_blue.photos.inspect