and 
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Rails 3.0 has been underway for a good two years, so it’s with immense pleasure that we can declare it’s finally here. We’ve brought the work of more than 1,600 contributors together to make everything better, faster, cleaner, and more beautiful.
This third generation of Rails has seen thousands of commits, so picking what to highlight was always going to be tough and incomplete. But here’s a choice selection of major changes for Rails 3:
New Active Record query engineActive Record has adopted the ARel query engine to make scopes and queries more consistent and composable. This makes it much easier to build complex queries over several iterations. We also delay the actual execution of the query until it’s needed. Here’s a simple example:
Read more in new Active Record guide and watch the Dive into Rails 3: ARel video.
New router for Action ControllerWhen we switched to a REST-based approach for controllers in Rails 2, we patched on the syntax to the existing router while we were waiting to see if the experiment panned out.
It did and for Rails 3 we’ve gone back and revamped the syntax completely to favor the REST style with less noise and more flexibility:
Read more in the new routing guide.
New Action MailerAction Mailer was born with a split-personality of half model, half controller. In Rails 3, we’ve made the choice to make it all controller. This means that the feel and functionality will be much closer to Action Controller and in fact they now share a bunch of underlying code. Here’s a taste of what it looks like now:
The new Action Mailer is built on top of the new Mail gem as well. Say goodbye to TMail headaches.
Read more in new Action Mailer guide.
Manage dependencies with BundlerManaging all the dependencies of a Rails application has long been a hassle of patchworks. We had config.gem, Capistrano externals, custom rake setup tasks, and other incomplete solutions.
Bundler cleans all that up and allows you to specify the libraries, frameworks, and plugins that your application depends on. All Rails 3 applications are born with a Gemfile to control it all. See more on the Bundler site.
XSS protection by defaultThe internet is a scary place and Rails 3 is watching out for you by default. We’ve had CRSF protection with form signing for a while and SQL-injection protection since the beginning, but Rails 3 ups the anté with XSS protection as well (hat tip to Django for convincing us).
See the Railscast on XSS video and the Dive into Rails 3: Cross-site scripting video for more.
Say goodbye to encoding issuesIf you browse the Internet with any frequency, you will likely encounter the � character. This problem is extremely pervasive, and is caused by mixing and matching content with different encodings.
In a system like Rails, content comes from the database, your templates, your source files, and from the user. Ruby 1.9 gives us the raw tools to eliminate these problems, and in combination with Rails 3, � should be a thing of the past in Rails applications. Never struggle with corrupted data pasted by a user from Microsoft Word again!
Active Model: ... Read more
