O/R MapperであるDataMapperの遅延評価の仕組みのサンプル

sample.rb

#!/bin/env ruby
# encoding: utf-8
# Author: kimoto
require 'data_mapper'

class Channel
  include DataMapper::Resource
  property :id, Serial
  property :name, String
end

DataMapper.setup(:default, "mysql://127.0.0.1/sample")
DataMapper.finalize
DataMapper.auto_upgrade!


100.times{ |n|
  Channel.create(:name => "ch#{n}")
}

DataMapper.logger.set_log STDERR, :debug, "", true # ログ出力を有効化
channels = Channel.all
channels.each{ |ch|
  ch.name # nop
}

# このとき実際に発行されるSQLは以下のみ
# (0.000365) SELECT `id`, `name` FROM `channels` ORDER BY `id`