class Photo
  include DataMapper::Resource

  property :id, Serial

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

class Tag
  include DataMapper::Resource

  property :id, Serial

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

class Tagging
  include DataMapper::Resource

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