Chinaonrails Rubyonrails21 Zh

This document provides an introduction to Ruby on Rails 2.1, including new features such as timezones, dirty tracking, gem dependencies, named scopes, UTC-based migrations, and better caching. It summarizes changes to ActiveRecord methods like sum and new options for has_one and has_many associations. The document is a translation of the original English book into Chinese by volunteers from the China on Rails community.