data_mapper_example.rb

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

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite::memory:')

class AttachmentTag
  include DataMapper::Resource
  property :id, Serial

  belongs_to :attachment
  belongs_to :tag
end

class Attachment
  include DataMapper::Resource
  property :id, Serial
  
  property :filename, String
  has n, :attachment_tags
  has n, :tags, :through => :attachment_tags
end

class Tag
  include DataMapper::Resource
  property :id, Serial

  property :tagtext, String
  has n, :attachment_tags
  has n, :attachments, :through => :attachment_tags
end

DataMapper.auto_migrate!

10.times do |n|
  Tag.create :tagtext => "tag##{n}"
end

10.times do |n|
  a = Attachment.create :filename => "attachment##{n}"
  Tag.all.each do |t|
    AttachmentTag.create :attachment => a, :tag => t
  end
end

Attachment.all.each do |a|
  a.attachment_tags.each do |at|
    puts "#{a.filename} is tagged with #{at.tag.tagtext}"
  end
end