If you're upgrading an existing application, it's a great idea to have good test coverage before going in. You should also first upgrade to Rails 3.1 in case you haven't and make sure your application still runs as expected before attempting an update to Rails 3.2. Then take heed of the following changes:
Rails 3.2 requires Ruby 1.8.7 or higher. Support for all of the previous Ruby versions has been dropped officially and you should upgrade as early as possible. Rails 3.2 is also compatible with Ruby 1.9.2.
TIP: Note that Ruby 1.8.7 p248 and p249 have marshalling bugs that crash Rails. Ruby Enterprise Edition has these fixed since the release of 1.8.7-2010.02. On the 1.9 front, Ruby 1.9.1 is not usable because it outright segfaults, so if you want to use 1.9.x, jump on to 1.9.2 or 1.9.3 for smooth sailing.
* Rails 3.2 deprecates `vendor/plugins` and Rails 4.0 will remove them completely. You can start replacing these plugins by extracting them as gems and adding them in your Gemfile. If you choose not to make them gems, you can move them into, say, `lib/my_plugin/*` and add an appropriate initializer in `config/initializers/my_plugin.rb`.
Rails now uses a `Gemfile` in the application root to determine the gems you require for your application to start. This `Gemfile` is processed by the [Bundler](https://github.com/carlhuda/bundler) gem, which then installs all your dependencies. It can even install all the dependencies locally to your application so that it doesn't depend on the system gems.
`Bundler` and `Gemfile` makes freezing your Rails application easy as pie with the new dedicated `bundle` command. If you want to bundle straight from the Git repository, you can pass the `--edge` flag:
Rails 3.2 comes with a development mode that's noticeably faster. Inspired by [Active Reload](https://github.com/paneq/active_reload), Rails reloads classes only when files actually change. The performance gains are dramatic on a larger application. Route recognition also got a bunch faster thanks to the new [Journey](https://github.com/rails/journey) engine.
Rails 3.2 comes with a nice feature that explains queries generated by Arel by defining an `explain` method in `ActiveRecord::Relation`. For example, you can run something like `puts Person.active.limit(5).explain` and the query Arel produces is explained. This allows to check for the proper indexes and further optimizations.
When running a multi-user, multi-account application, it's a great help to be able to filter the log by who did what. TaggedLogging in Active Support helps in doing exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.
* Speed up development by only reloading classes if dependencies files changed. This can be turned off by setting `config.reload_classes_only_on_change` to false.
* New applications get a flag `config.active_record.auto_explain_threshold_in_seconds` in the environments configuration files. With a value of `0.5` in `development.rb` and commented out in `production.rb`. No mention in `test.rb`.
* Added `config.exceptions_app` to set the exceptions application invoked by the `ShowException` middleware when an exception happens. Defaults to `ActionDispatch::PublicExceptions.new(Rails.public_path)`.
* Update `Rails::Rack::Logger` middleware to apply any tags set in `config.log_tags` to `ActiveSupport::TaggedLogging`. This makes it easy to tag log lines with debug information like subdomain and request id -- both very helpful in debugging multi-user production applications.
* Default options to `rails new` can be set in `~/.railsrc`. You can specify extra command-line arguments to be used every time `rails new` runs in the `.railsrc` configuration file in your home directory.
will create indexes for `title` and `author` with the latter being a unique index. Some types such as decimal accept custom options. In the example, `price` will be a decimal column with precision and scale set to 7 and 2 respectively.
*`Rails::Plugin` is deprecated and will be removed in Rails 4.0. Instead of adding plugins to `vendor/plugins` use gems or bundler with path or git dependencies.
* Make `ActiveSupport::Benchmarkable` a default module for `ActionController::Base,` so the `#benchmark` method is once again available in the controller context like it used to be.
* Rails will now use your default layout (such as "layouts/application") when you specify a layout with `:only` and `:except` condition, and those conditions fail.
Rails will use `layouts/single_car` when a request comes in `:show` action, and use `layouts/application` (or `layouts/cars`, if exists) when a request comes in for any other actions.
*`ActionController::ParamsWrapper` on Active Record models now only wrap `attr_accessible` attributes if they were set. If not, only the attributes returned by the class method `attribute_names` will be wrapped. This fixes the wrapping of nested attributes by adding them to `attr_accessible`.
*`ActionDispatch::ShowExceptions` is refactored. The controller is responsible for choosing to show exceptions. It's possible to override `show_detailed_exceptions?` in controllers to specify which requests should provide debugging information on errors.
We now no longer write out HTTP_COOKIE and the cookie jar is persistent between requests so if you need to manipulate the environment for your test you need to do it before the cookie jar is created.
In the example above, `PostsController` will no longer automatically look up for a posts layout. If you need this functionality you could either remove `layout "application"` from `ApplicationController` or explicitly set it to `nil` in `PostsController`.
* Added `ActionDispatch::RequestId` middleware that'll make a unique X-Request-Id header available to the response and enables the `ActionDispatch::Request#uuid` method. This makes it easy to trace requests from end-to-end in the stack and to identify individual requests in mixed logs like Syslog.
* The `ShowExceptions` middleware now accepts an exceptions application that is responsible to render an exception when the application fails. The application is invoked with a copy of the exception in `env["action_dispatch.exception"]` and with the `PATH_INFO` rewritten to the status code.
* Date helpers accept a new option `:use_two_digit_numbers => true`, that renders select boxes for months and days with a leading zero without changing the respective values. For example, this is useful for displaying ISO 8601-style dates such as '2011-08-01'.
* You can provide a namespace for your form to ensure uniqueness of id attributes on form elements. The namespace attribute will be prefixed with underscore on the generated HTML id.
*`content_tag_for` and `div_for` can now take a collection of records. It will also yield the record as the first argument if you set a receiving argument in your block. So instead of having to do this:
* Passing formats or handlers to render :template and friends like `render :template => "foo.html.erb"` is deprecated. Instead, you can provide :handlers and :formats directly as options: ` render :template => "foo", :formats => [:html, :js], :handlers => :erb`.
* Adds a configuration option `config.assets.logger` to control Sprockets logging. Set it to `false` to turn off logging and to `nil` to default to `Rails.logger`.
* Implements automatic EXPLAIN logging for slow queries. A new configuration parameter `config.active_record.auto_explain_threshold_in_seconds` determines what's to be considered a slow query. Setting that to nil disables this feature. Defaults are 0.5 in development mode, and nil in test and production modes. Rails 3.2 supports this feature in SQLite, MySQL (mysql2 adapter), and PostgreSQL.
* Added ability to run migrations only for a given scope, which allows to run migrations only from one engine (for example to revert changes from an engine that need to be removed).
* Implemented `ActiveRecord::Relation#pluck` method that returns an array of column values directly from the underlying table. This also works with serialized attributes.
* Generated association methods are created within a separate module to allow overriding and composition. For a class named MyModel, the module is named `MyModel::GeneratedFeatureMethods`. It is included into the model class immediately after the `generated_attributes_methods` module defined in Active Model, so association methods override attribute methods of the same name.
* Allow the `:class_name` option for associations to take a symbol in addition to a string. This is to avoid confusing newbies, and to be consistent with the fact that other options like `:foreign_key` already allow a symbol or a string.
* Case-insensitive uniqueness validation avoids calling LOWER in MySQL when the column already uses a case-insensitive collation.
* Transactional fixtures enlist all active database connections. You can test models on different connections without disabling transactional fixtures.
* Add `first_or_create`, `first_or_create!`, `first_or_initialize` methods to Active Record. This is a better approach over the old `find_or_create_by` dynamic methods because it's clearer which arguments are used to find the record and which are used to create it.
* Added a `with_lock` method to Active Record objects, which starts a transaction, locks the object (pessimistically) and yields to the block. The method takes one (optional) parameter and passes it to `lock!`.
* The `set_table_name`, `set_inheritance_column`, `set_sequence_name`, `set_primary_key`, `set_locking_column` methods are deprecated. Use an assignment method instead. For example, instead of `set_table_name`, use `self.table_name=`.
* Provide mass_assignment_sanitizer as an easy API to replace the sanitizer behavior. Also support both :logger (default) and :strict sanitizer behavior.
* Deprecated `define_attr_method` in `ActiveModel::AttributeMethods` because this only existed to support methods like `set_table_name` in Active Record, which are themselves being deprecated.
* The `beginning_of_week` method in `Date`, `Time` and `DateTime` accepts an optional argument representing the day in which the week is assumed to start.
* Defined new methods `Module#qualified_const_defined?`, `Module#qualified_const_get` and `Module#qualified_const_set` that are analogous to the corresponding methods in the standard API, but accept qualified constant names.
* Added `safe_constantize` that constantizes a string but returns `nil` instead of raising an exception if the constant (or part of it) does not exist.
* The definition of a blank string for Ruby 1.9 has been extended to Unicode whitespace. Also, in Ruby 1.8 the ideographic space U`3000 is considered to be whitespace.
*`ActiveSupport::BufferedLogger's` behavior of automatically creating the directory for your log file is deprecated. Please make sure to create the directory for your log file before instantiating.
*`ActiveSupport::BufferedLogger#auto_flushing` is deprecated. Either set the sync level on the underlying file handle like this. Or tune your filesystem. The FS cache is now what controls flushing.
See the [full list of contributors to Rails](http://contributors.rubyonrails.org/) for the many people who spent many hours making Rails, the stable and robust framework it is. Kudos to all of them.
