1. Upgrading to Rails 4.0

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 theUpgrading Ruby on Rails guide.

2. Creating a Rails 4.0 application

# You should have the 'rails' RubyGem installed$railsnew myapp$cdmyapp

2.1. Vendoring Gems

Rails now uses aGemfile in the application root to determine the gems you require for your application to start. ThisGemfile is processed by theBundler 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

2.2. Living on the Edge

Bundler andGemfile makes freezing your Rails application easy as pie with the new dedicatedbundle command. If you want to bundle straight from the Git repository, you can pass the--edge flag:

$railsnew 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

3. Major Features

3.1. Upgrade

• Ruby 1.9.3 (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.
• 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.
• 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.
• 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.
• ActiveModel mass assignment protection (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.
• vendor/plugins removed (commit) - Use aGemfile to manage installed gems.

3.2. ActionPack

• Strong parameters (commit) - Only allow permitted parameters to update model objects (params.permit(:title, :text)).
• Routing concerns (commit) - In the routing DSL, factor out common subroutes (comments from/posts/1/comments and/videos/1/comments).
• ActionController::Live (commit) - Stream JSON withresponse.stream.
• Declarative ETags (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.
• 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.
• Decouple ActionView from ActionController (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.

3.3. General

• ActiveModel::Model (commit) -ActiveModel::Model, a mixin to make normal Ruby objects to work with ActionPack out of box (ex. forform_for)
• New scope API (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.
• Support for specifying transaction isolation level (commit) - Choose whether repeatable reads or improved performance (less locking) is more important.
• Dalli (commit) - Use Dalli memcache client for the memcache store.
• Notifications start & finish (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.

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

3.4. Security

• match do not catch all (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 withraw orhtml_safe 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) andX-Content-Type-Options (prevents the browser from opening a jpeg as an exe).

4. Extraction of features to gems

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

• Hash-based & Dynamic finder methods (GitHub)
• Mass assignment protection in Active Record models (GitHub,Pull Request)
• ActiveRecord::SessionStore (GitHub,Pull Request)
• Active Record Observers (GitHub,Commit)
• Active Resource (GitHub,Pull Request,Blog)
• Action Caching (GitHub,Pull Request)
• Page Caching (GitHub,Pull Request)
• Sprockets (GitHub)
• Performance tests (GitHub,Pull Request)

5. Documentation

• Guides are rewritten in GitHub Flavored Markdown.

• Guides have a responsive design.

6. Railties

Please refer to theChangelog for detailed changes.

6.1. Notable changes

• New test locationstest/models,test/helpers,test/controllers, andtest/mailers. Corresponding rake tasks added as well. (Pull Request)

• Your app's executables now live in thebin/ directory. Runrake rails:update:bin to getbin/bundle,bin/rails, andbin/rake.

• Threadsafe on by default

• Ability to use a custom builder by passing--builder (or-b) torails new has been removed. Consider using application templatesinstead. (Pull Request)

6.2. Deprecations

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

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

7. Action Mailer

Please refer to theChangelog for detailed changes.

7.1. Notable changes

7.2. Deprecations

8. Active Model

Please refer to theChangelog for detailed changes.

8.1. Notable changes

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

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

8.2. Deprecations

9. Active Support

Please refer to theChangelog for detailed changes.

9.1. Notable changes

• Replace deprecatedmemcache-client gem withdalli inActiveSupport::Cache::MemCacheStore.

• OptimizeActiveSupport::Cache::Entry to reduce memory and processing overhead.

• Inflections can now be defined per locale.singularize andpluralize 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 newObject#try!.

• String#to_date now raisesArgumentError: invalid date instead ofNoMethodError: undefined method 'div' for nil:NilClasswhen given an invalid date. It is now the same asDate.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

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9.2. Deprecations

• DeprecateActiveSupport::TestCase#pending method, useskip 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 methodModule#local_constant_names, useModule#local_constants instead (which returns symbols).

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

• Deprecateassert_present andassert_blank in favor ofassert object.blank? andassert object.present?

10. Action Pack

Please refer to theChangelog for detailed changes.

10.1. Notable 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.

10.2. Deprecations

11. Active Record

Please refer to theChangelog for detailed changes.

11.1. Notable changes

• Improve ways to writechange migrations, making the oldup &down methods no longer necessary.

  • The methodsdrop_table andremove_column are now reversible, as long as the necessary information is given.The methodremove_column used to accept multiple column names; instead useremove_columns (which is not revertible).The methodchange_table is also reversible, as long as its block doesn't callremove,change orchange_default
  • New methodreversible makes it possible to specify code to be run when migrating up or down.See theGuide on Migration
  • New methodrevert will revert a whole migration or the given block.If migrating down, the given migration / block is run normally.See theGuide on Migration

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

• AddRelation#load to explicitly load the record and returnself.

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

• AddedActiveRecord::Migration.check_pending! that raises an error if migrations are pending.

• Added custom coders support forActiveRecord::Store. Now you can set your custom coder like this:

  store :settings, accessors: [ :color, :homepage ], coder: JSON

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• mysql andmysql2 connections will setSQL_MODE=STRICT_ALL_TABLES by default to avoid silent data loss. This can be disabled by specifyingstrict: false in yourdatabase.yml.

• Remove IdentityMap.

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

• AddsActiveRecord::NullRelation andActiveRecord::Relation#none implementing the null object pattern for the Relation class.

• Addedcreate_join_table migration helper to create HABTM join tables.

• Allows PostgreSQL hstore records to be created.

11.2. Deprecations

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

• All dynamic methods except forfind_by_... andfind_by_...! are deprecated. Here'show you can rewrite the code:

  • find_all_by_... can be rewritten usingwhere(...).
  • find_last_by_... can be rewritten usingwhere(...).last.
  • scoped_by_... can be rewritten usingwhere(...).
  • find_or_initialize_by_... can be rewritten usingfind_or_initialize_by(...).
  • find_or_create_by_... can be rewritten usingfind_or_create_by(...).
  • find_or_create_by_...! can be rewritten usingfind_or_create_by!(...).

12. Credits

See thefull 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.