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 3.2 in case you haven't and make sure your application still runs as expected before attempting an update to Rails 4.0. A list of things to watch out for when upgrading is available in the Upgrading Ruby on Rails guide.

# You should have the 'rails' RubyGem installed $ rails new myapp $ cd myapp

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 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.

More information: Bundler homepage

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 new myapp --edge

If you have a local checkout of the Rails repository and want to generate an application using that, you can pass the --dev flag:

$ ruby /path/to/rails/railties/bin/rails new myapp --dev

Ruby 1.9.3 (commit) - Ruby 2.0 preferred; 1.9.3+ required

(commit) - Ruby 2.0 preferred; 1.9.3+ required New deprecation policy - Deprecated features are warnings in Rails 4.0 and will be removed in Rails 4.1.

- Deprecated features are warnings in Rails 4.0 and will be removed in Rails 4.1. ActionPack page and action caching (commit) - Page and action caching are extracted to a separate gem. Page and action caching requires too much manual intervention (manually expiring caches when the underlying model objects are updated). Instead, use Russian doll caching.

(commit) - Page and action caching are extracted to a separate gem. Page and action caching requires too much manual intervention (manually expiring caches when the underlying model objects are updated). Instead, use Russian doll caching. ActiveRecord observers (commit) - Observers are extracted to a separate gem. Observers are only needed for page and action caching, and can lead to spaghetti code.

(commit) - Observers are extracted to a separate gem. Observers are only needed for page and action caching, and can lead to spaghetti code. ActiveRecord session store (commit) - The ActiveRecord session store is extracted to a separate gem. Storing sessions in SQL is costly. Instead, use cookie sessions, memcache sessions, or a custom session store.

(commit) - The ActiveRecord session store is extracted to a separate gem. Storing sessions in SQL is costly. Instead, use cookie sessions, memcache sessions, or a custom session store. ActiveModel mass assignment protection (commit) - Rails 3 mass assignment protection is deprecated. Instead, use strong parameters.

(commit) - Rails 3 mass assignment protection is deprecated. Instead, use strong parameters. ActiveResource (commit) - ActiveResource is extracted to a separate gem. ActiveResource was not widely used.

(commit) - ActiveResource is extracted to a separate gem. ActiveResource was not widely used. vendor/plugins removed (commit) - Use a Gemfile to manage installed gems.

Strong parameters (commit) - Only allow permitted parameters to update model objects ( params.permit(:title, :text) ).

(commit) - Only allow permitted parameters to update model objects ( ). Routing concerns (commit) - In the routing DSL, factor out common subroutes ( comments from /posts/1/comments and /videos/1/comments ).

(commit) - In the routing DSL, factor out common subroutes ( from and ). ActionController::Live (commit) - Stream JSON with response.stream .

(commit) - Stream JSON with . Declarative ETags (commit) - Add controller-level etag additions that will be part of the action etag computation.

(commit) - Add controller-level etag additions that will be part of the action etag computation. Russian doll caching (commit) - Cache nested fragments of views. Each fragment expires based on a set of dependencies (a cache key). The cache key is usually a template version number and a model object.

(commit) - Cache nested fragments of views. Each fragment expires based on a set of dependencies (a cache key). The cache key is usually a template version number and a model object. Turbolinks (commit) - Serve only one initial HTML page. When the user navigates to another page, use pushState to update the URL and use AJAX to update the title and body.

(commit) - Serve only one initial HTML page. When the user navigates to another page, use pushState to update the URL and use AJAX to update the title and body. Decouple ActionView from ActionController (commit) - ActionView was decoupled from ActionPack and will be moved to a separated gem in Rails 4.1.

(commit) - ActionView was decoupled from ActionPack and will be moved to a separated gem in Rails 4.1. Do not depend on ActiveModel (commit) - ActionPack no longer depends on ActiveModel.

ActiveModel::Model (commit) - ActiveModel::Model , a mixin to make normal Ruby objects to work with ActionPack out of box (ex. for form_for )

(commit) - , a mixin to make normal Ruby objects to work with ActionPack out of box (ex. for ) New scope API (commit) - Scopes must always use callables.

(commit) - Scopes must always use callables. Schema cache dump (commit) - To improve Rails boot time, instead of loading the schema directly from the database, load the schema from a dump file.

(commit) - To improve Rails boot time, instead of loading the schema directly from the database, load the schema from a dump file. Support for specifying transaction isolation level (commit) - Choose whether repeatable reads or improved performance (less locking) is more important.

(commit) - Choose whether repeatable reads or improved performance (less locking) is more important. Dalli (commit) - Use Dalli memcache client for the memcache store.

(commit) - Use Dalli memcache client for the memcache store. Notifications start & finish (commit) - Active Support instrumentation reports start and finish notifications to subscribers.

(commit) - Active Support instrumentation reports start and finish notifications to subscribers. Thread safe by default (commit) - Rails can run in threaded app servers without additional configuration.

Check that the gems you are using are threadsafe.

PATCH verb (commit) - In Rails, PATCH replaces PUT. PATCH is used for partial updates of resources.

match do not catch all (commit) - In the routing DSL, match requires the HTTP verb or verbs to be specified.

(commit) - In the routing DSL, match requires the HTTP verb or verbs to be specified. html entities escaped by default (commit) - Strings rendered in erb are escaped unless wrapped with raw or html_safe is called.

(commit) - Strings rendered in erb are escaped unless wrapped with or is called. New security headers (commit) - Rails sends the following headers with every HTTP request: X-Frame-Options (prevents clickjacking by forbidding the browser from embedding the page in a frame), X-XSS-Protection (asks the browser to halt script injection) and X-Content-Type-Options (prevents the browser from opening a jpeg as an exe).

In Rails 4.0, several features have been extracted into gems. You can simply add the extracted gems to your Gemfile to bring the functionality back.

Guides are rewritten in GitHub Flavored Markdown.

Guides have a responsive design.

Please refer to the Changelog for detailed changes.

New test locations test/models , test/helpers , test/controllers , and test/mailers . Corresponding rake tasks added as well. (Pull Request)

Your app's executables now live in the bin/ directory. Run rake rails:update:bin to get bin/bundle , bin/rails , and bin/rake .

Threadsafe on by default

Ability to use a custom builder by passing --builder (or -b ) to rails new has been removed. Consider using application templates instead. (Pull Request)

config.threadsafe! is deprecated in favor of config.eager_load which provides a more fine grained control on what is eager loaded.

Rails::Plugin has gone. Instead of adding plugins to vendor/plugins use gems or bundler with path or git dependencies.

Please refer to the Changelog for detailed changes.

Please refer to the Changelog for detailed changes.

Add ActiveModel::ForbiddenAttributesProtection , a simple module to protect attributes from mass assignment when non-permitted attributes are passed.

Added ActiveModel::Model , a mixin to make Ruby objects work with Action Pack out of box.

Please refer to the Changelog for detailed changes.

Replace deprecated memcache-client gem with dalli in ActiveSupport::Cache::MemCacheStore .

Optimize ActiveSupport::Cache::Entry to reduce memory and processing overhead.

Inflections can now be defined per locale. singularize and pluralize accept locale as an extra argument.

Object#try will now return nil instead of raise a NoMethodError if the receiving object does not implement the method, but you can still get the old behavior by using the new Object#try! .

String#to_date now raises ArgumentError: invalid date instead of NoMethodError: undefined method 'div' for nil:NilClass when given an invalid date. It is now the same as Date.parse , and it accepts more invalid dates than 3.x, such as:

# ActiveSupport 3.x "asdf" . to_date # => NoMethodError: undefined method `div' for nil:NilClass "333" . to_date # => NoMethodError: undefined method `div' for nil:NilClass # ActiveSupport 4 "asdf" . to_date # => ArgumentError: invalid date "333" . to_date # => Fri, 29 Nov 2013

Deprecate ActiveSupport::TestCase#pending method, use skip from minitest instead.

ActiveSupport::Benchmarkable#silence has been deprecated due to its lack of thread safety. It will be removed without replacement in Rails 4.1.

ActiveSupport::JSON::Variable is deprecated. Define your own #as_json and #encode_json methods for custom JSON string literals.

Deprecates the compatibility method Module#local_constant_names , use Module#local_constants instead (which returns symbols).

BufferedLogger is deprecated. Use ActiveSupport::Logger , or the logger from Ruby standard library.

Deprecate assert_present and assert_blank in favor of assert object.blank? and assert object.present?

Please refer to the Changelog for detailed changes.

Change the stylesheet of exception pages for development mode. Additionally display also the line of code and fragment that raised the exception in all exceptions pages.

Please refer to the Changelog for detailed changes.

Improve ways to write change migrations, making the old up & down methods no longer necessary. The methods drop_table and remove_column are now reversible, as long as the necessary information is given. The method remove_column used to accept multiple column names; instead use remove_columns (which is not revertible). The method change_table is also reversible, as long as its block doesn't call remove , change or change_default New method reversible makes it possible to specify code to be run when migrating up or down. See the Guide on Migration New method revert will revert a whole migration or the given block. If migrating down, the given migration / block is run normally. See the Guide on Migration

Adds PostgreSQL array type support. Any datatype can be used to create an array column, with full migration and schema dumper support.

Add Relation#load to explicitly load the record and return self .

Model.all now returns an ActiveRecord::Relation , rather than an array of records. Use Relation#to_a if you really want an array. In some specific cases, this may cause breakage when upgrading.

Added ActiveRecord::Migration.check_pending! that raises an error if migrations are pending.

Added custom coders support for ActiveRecord::Store . Now you can set your custom coder like this: store :settings, accessors: [ :color, :homepage ], coder: JSON

mysql and mysql2 connections will set SQL_MODE=STRICT_ALL_TABLES by default to avoid silent data loss. This can be disabled by specifying strict: false in your database.yml .

Remove IdentityMap.

Remove automatic execution of EXPLAIN queries. The option active_record.auto_explain_threshold_in_seconds is no longer used and should be removed.

Adds ActiveRecord::NullRelation and ActiveRecord::Relation#none implementing the null object pattern for the Relation class.

Added create_join_table migration helper to create HABTM join tables.

Allows PostgreSQL hstore records to be created.

Deprecated the old-style hash based finder API. This means that methods which previously accepted "finder options" no longer do.

All dynamic methods except for find_by_... and find_by_...! are deprecated. Here's how you can rewrite the code: find_all_by_... can be rewritten using where(...) . find_last_by_... can be rewritten using where(...).last . scoped_by_... can be rewritten using where(...) . find_or_initialize_by_... can be rewritten using find_or_initialize_by(...) . find_or_create_by_... can be rewritten using find_or_create_by(...) . find_or_create_by_...! can be rewritten using find_or_create_by!(...) .



See the full list of contributors to Rails for the many people who spent many hours making Rails, the stable and robust framework it is. Kudos to all of them.

Feedback

You're encouraged to help improve the quality of this guide.

Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.

You may also find incomplete content or stuff that is not up to date. Please do add any missing documentation for master. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the master branch. Check the Ruby on Rails Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome on the rubyonrails-docs mailing list.