A Guide for Upgrading Ruby on Rails

1 General Advice

Before attempting to upgrade an existing application, you should be sure you have a good reason to upgrade. You need to balance out several factors: the need for new features, the increasing difficulty of finding support for old code, and your available time and skills, to name a few.

1.1 Test Coverage

The best way to be sure that your application still works after upgrading is to have good test coverage before you start the process. If you don't have automated tests that exercise the bulk of your application, you'll need to spend time manually exercising all the parts that have changed. In the case of a Rails upgrade, that will mean every single piece of functionality in the application. Do yourself a favor and make sure your test coverage is good before you start an upgrade.

1.2 Ruby Versions

Rails generally stays close to the latest released Ruby version when it's released:

Rails 3 and above require Ruby 1.8.7 or higher. Support for all of the previous Ruby versions has been dropped officially. You should upgrade as early as possible.
Rails 3.2.x is the last branch to support Ruby 1.8.7.
Rails 4 prefers Ruby 2.0 and requires 1.9.3 or newer.

Ruby 1.8.7 p248 and p249 have marshaling 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 straight to 1.9.3 for smooth sailing.

2 Upgrading from Rails 3.2 to Rails 4.0

This section is a work in progress.

If your application is currently on any version of Rails older than 3.2.x, you should upgrade to Rails 3.2 before attempting one to Rails 4.0.

The following changes are meant for upgrading your application to Rails 4.0.

2.1 Gemfile

Rails 4.0 removed the assets group from Gemfile. You'd need to remove that line from your Gemfile when upgrading. You should also update your application file (in config/application.rb):

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(:default, Rails.env)

2.2 vendor/plugins

Rails 4.0 no longer supports loading plugins from vendor/plugins. You must replace any plugins by extracting them to gems and adding them to 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.

2.3 Active Record

Rails 4.0 has removed the identity map from Active Record, due to some inconsistencies with associations. If you have manually enabled it in your application, you will have to remove the following config that has no effect anymore: config.active_record.identity_map.
The delete method in collection associations can now receive Fixnum or String arguments as record ids, besides records, pretty much like the destroy method does. Previously it raised ActiveRecord::AssociationTypeMismatch for such arguments. From Rails 4.0 on delete automatically tries to find the records matching the given ids before deleting them.
In Rails 4.0 when a column or a table is renamed the related indexes are also renamed. If you have migrations which rename the indexes, they are no longer needed.
Rails 4.0 has changed serialized_attributes and attr_readonly to class methods only. You shouldn't use instance methods since it's now deprecated. You should change them to use class methods, e.g. self.serialized_attributes to self.class.serialized_attributes.
When using the default coder, assigning nil to a serialized attribute will save it to the database as NULL instead of passing the nil value through YAML ("--- \n...\n").
Rails 4.0 has removed attr_accessible and attr_protected feature in favor of Strong Parameters. You can use the Protected Attributes gem for a smooth upgrade path.
If you are not using Protected Attributes, you can remove any options related to this gem such as whitelist_attributes or mass_assignment_sanitizer options.
Rails 4.0 requires that scopes use a callable object such as a Proc or lambda:

  scope :active, where(active: true)

  # becomes
  scope :active, -> { where active: true }

Rails 4.0 has deprecated ActiveRecord::Fixtures in favor of ActiveRecord::FixtureSet.
Rails 4.0 has deprecated ActiveRecord::TestCase in favor of ActiveSupport::TestCase.
Rails 4.0 has 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 handle the changes:
- find_all_by_... becomes where(...).
- find_last_by_... becomes where(...).last.
- scoped_by_... becomes where(...).
- find_or_initialize_by_... becomes find_or_initialize_by(...).
- find_or_create_by_... becomes find_or_create_by(...).

2.4 Active Resource

Rails 4.0 extracted Active Resource to its own gem. If you still need the feature you can add the Active Resource gem in your Gemfile.

2.5 Active Model

Rails 4.0 has changed how errors attach with the ActiveModel::Validations::ConfirmationValidator. Now when confirmation validations fail, the error will be attached to :#{attribute}_confirmation instead of attribute.
Rails 4.0 has changed ActiveModel::Serializers::JSON.include_root_in_json default value to false. Now, Active Model Serializers and Active Record objects have the same default behaviour. This means that you can comment or remove the following option in the config/initializers/wrap_parameters.rb file:

# Disable root element in JSON by default.
# ActiveSupport.on_load(:active_record) do
#   self.include_root_in_json = false
# end

2.6 Action Pack

Rails 4.0 introduces ActiveSupport::KeyGenerator and uses this as a base from which to generate and verify signed cookies (among other things). Existing signed cookies generated with Rails 3.x will be transparently upgraded if you leave your existing secret_token in place and add the new secret_key_base.

  # config/initializers/secret_token.rb
  Myapp::Application.config.secret_token = 'existing secret token'
  Myapp::Application.config.secret_key_base = 'new secret key base'

Please note that you should wait to set secret_key_base until you have 100% of your userbase on Rails 4.x and are reasonably sure you will not need to rollback to Rails 3.x. This is because cookies signed based on the new secret_key_base in Rails 4.x are not backwards compatible with Rails 3.x. You are free to leave your existing secret_token in place, not set the new secret_key_base, and ignore the deprecation warnings until you are reasonably sure that your upgrade is otherwise complete.

If you are relying on the ability for external applications or Javascript to be able to read your Rails app's signed session cookies (or signed cookies in general) you should not set secret_key_base until you have decoupled these concerns.

Rails 4.0 encrypts the contents of cookie-based sessions if secret_key_base has been set. Rails 3.x signed, but did not encrypt, the contents of cookie-based session. Signed cookies are "secure" in that they are verified to have been generated by your app and are tamper-proof. However, the contents can be viewed by end users, and encrypting the contents eliminates this caveat/concern without a significant performance penalty.

Please read Pull Request #9978 for details on the move to encrypted session cookies.

Rails 4.0 removed the ActionController::Base.asset_path option. Use the assets pipeline feature.
Rails 4.0 has deprecated ActionController::Base.page_cache_extension option. Use ActionController::Base.default_static_extension instead.
Rails 4.0 has removed Action and Page caching from Action Pack. You will need to add the actionpack-action_caching gem in order to use caches_action and the actionpack-page_caching to use caches_pages in your controllers.
Rails 4.0 has removed the XML parameters parser. You will need to add the actionpack-xml_parser gem if you require this feature.
Rails 4.0 changes the default memcached client from memcache-client to dalli. To upgrade, simply add gem 'dalli' to your Gemfile.
Rails 4.0 deprecates the dom_id and dom_class methods in controllers (they are fine in views). You will need to include the ActionView::RecordIdentifier module in controllers requiring this feature.
Rails 4.0 deprecates the :confirm option for the link_to helper. You should instead rely on a data attribute (e.g. data: { confirm: 'Are you sure?' }). This deprecation also concerns the helpers based on this one (such as link_to_if or link_to_unless).
Rails 4.0 changed how assert_generates, assert_recognizes, and assert_routing work. Now all these assertions raise Assertion instead of ActionController::RoutingError.
Rails 4.0 raises an ArgumentError if clashing named routes are defined. This can be triggered by explicitly defined named routes or by the resources method. Here are two examples that clash with routes named example_path:

  get 'one' => 'test#example', as: :example
  get 'two' => 'test#example', as: :example

  resources :examples
  get 'clashing/:id' => 'test#example', as: :example

In the first case, you can simply avoid using the same name for multiple routes. In the second, you can use the only or except options provided by the resources method to restrict the routes created as detailed in the Routing Guide.

Rails 4.0 also changed the way unicode character routes are drawn. Now you can draw unicode character routes directly. If you already draw such routes, you must change them, for example:

get Rack::Utils.escape('こんにちは'), controller: 'welcome', action: 'index'

becomes

get 'こんにちは', controller: 'welcome', action: 'index'

Rails 4.0 requires that routes using match must specify the request method. For example:

  # Rails 3.x
  match "/" => "root#index"

  # becomes
  match "/" => "root#index", via: :get

  # or
  get "/" => "root#index"

Rails 4.0 has removed ActionDispatch::BestStandardsSupport middleware, <!DOCTYPE html> already triggers standards mode per http://msdn.microsoft.com/en-us/library/jj676915(v=vs.85).aspx and ChromeFrame header has been moved to config.action_dispatch.default_headers.

Remember you must also remove any references to the middleware from your application code, for example:

# Raise exception
config.middleware.insert_before(Rack::Lock, ActionDispatch::BestStandardsSupport)

Also check your environment settings for config.action_dispatch.best_standards_support and remove it if present.

In Rails 4.0, precompiling assets no longer automatically copies non-JS/CSS assets from vendor/assets and lib/assets. Rails application and engine developers should put these assets in app/assets or configure config.assets.precompile.
In Rails 4.0, ActionController::UnknownFormat is raised when the action doesn't handle the request format. By default, the exception is handled by responding with 406 Not Acceptable, but you can override that now. In Rails 3, 406 Not Acceptable was always returned. No overrides.
In Rails 4.0, a generic ActionDispatch::ParamsParser::ParseError exception is raised when ParamsParser fails to parse request params. You will want to rescue this exception instead of the low-level MultiJson::DecodeError, for example.
In Rails 4.0, SCRIPT_NAME is properly nested when engines are mounted on an app that's served from a URL prefix. You no longer have to set default_url_options[:script_name] to work around overwritten URL prefixes.
Rails 4.0 deprecated ActionController::Integration in favor of ActionDispatch::Integration.
Rails 4.0 deprecated ActionController::IntegrationTest in favor of ActionDispatch::IntegrationTest.
Rails 4.0 deprecated ActionController::PerformanceTest in favor of ActionDispatch::PerformanceTest.
Rails 4.0 deprecated ActionController::AbstractRequest in favor of ActionDispatch::Request.
Rails 4.0 deprecated ActionController::Request in favor of ActionDispatch::Request.
Rails 4.0 deprecated ActionController::AbstractResponse in favor of ActionDispatch::Response.
Rails 4.0 deprecated ActionController::Response in favor of ActionDispatch::Response.
Rails 4.0 deprecated ActionController::Routing in favor of ActionDispatch::Routing.

2.7 Active Support

Rails 4.0 removes the j alias for ERB::Util#json_escape since j is already used for ActionView::Helpers::JavaScriptHelper#escape_javascript.

2.8 Helpers Loading Order

The order in which helpers from more than one directory are loaded has changed in Rails 4.0. Previously, they were gathered and then sorted alphabetically. After upgrading to Rails 4.0, helpers will preserve the order of loaded directories and will be sorted alphabetically only within each directory. Unless you explicitly use the helpers_path parameter, this change will only impact the way of loading helpers from engines. If you rely on the ordering, you should check if correct methods are available after upgrade. If you would like to change the order in which engines are loaded, you can use config.railties_order= method.

2.9 Active Record Observer and Action Controller Sweeper

Active Record Observer and Action Controller Sweeper have been extracted to the rails-observers gem. You will need to add the rails-observers gem if you require these features.

2.10 sprockets-rails

assets:precompile:primary has been removed. Use assets:precompile instead.
The config.assets.compress option should be changed to config.assets.js_compressor like so for instance:

config.assets.js_compressor = :uglifier

2.11 sass-rails

asset_url with two arguments is deprecated. For example: asset-url("rails.png", image) becomes asset-url("rails.png")

3 Upgrading from Rails 3.1 to Rails 3.2

If your application is currently on any version of Rails older than 3.1.x, you should upgrade to Rails 3.1 before attempting an update to Rails 3.2.

The following changes are meant for upgrading your application to Rails 3.2.12, the latest 3.2.x version of Rails.

3.1 Gemfile

Make the following changes to your Gemfile.

gem 'rails', '= 3.2.12'

group :assets do
  gem 'sass-rails',   '~> 3.2.3'
  gem 'coffee-rails', '~> 3.2.1'
  gem 'uglifier',     '>= 1.0.3'
end

3.2 config/environments/development.rb

There are a couple of new configuration settings that you should add to your development environment:

# Raise exception on mass assignment protection for Active Record models
config.active_record.mass_assignment_sanitizer = :strict

# Log the query plan for queries taking more than this (works
# with SQLite, MySQL, and PostgreSQL)
config.active_record.auto_explain_threshold_in_seconds = 0.5

3.3 config/environments/test.rb

The mass_assignment_sanitizer configuration setting should also be be added to config/environments/test.rb:

# Raise exception on mass assignment protection for Active Record models
config.active_record.mass_assignment_sanitizer = :strict

3.4 vendor/plugins

Rails 3.2 deprecates vendor/plugins and Rails 4.0 will remove them completely. While it's not strictly necessary as part of a Rails 3.2 upgrade, you can start replacing any plugins by extracting them to gems and adding them to 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.

4 Upgrading from Rails 3.0 to Rails 3.1

If your application is currently on any version of Rails older than 3.0.x, you should upgrade to Rails 3.0 before attempting an update to Rails 3.1.

The following changes are meant for upgrading your application to Rails 3.1.11, the latest 3.1.x version of Rails.

4.1 Gemfile

Make the following changes to your Gemfile.

gem 'rails', '= 3.1.11'
gem 'mysql2'

# Needed for the new asset pipeline
group :assets do
  gem 'sass-rails',   "~> 3.1.5"
  gem 'coffee-rails', "~> 3.1.1"
  gem 'uglifier',     ">= 1.0.3"
end

# jQuery is the default JavaScript library in Rails 3.1
gem 'jquery-rails'

4.2 config/application.rb

The asset pipeline requires the following additions:

config.assets.enabled = true
config.assets.version = '1.0'

If your application is using an "/assets" route for a resource you may want change the prefix used for assets to avoid conflicts:

# Defaults to '/assets'
config.assets.prefix = '/asset-files'

4.3 config/environments/development.rb

Remove the RJS setting config.action_view.debug_rjs = true.

Add these settings if you enable the asset pipeline:

# Do not compress assets
config.assets.compress = false

# Expands the lines which load the assets
config.assets.debug = true

4.4 config/environments/production.rb

Again, most of the changes below are for the asset pipeline. You can read more about these in the Asset Pipeline guide.

# Compress JavaScripts and CSS
config.assets.compress = true

# Don't fallback to assets pipeline if a precompiled asset is missed
config.assets.compile = false

# Generate digests for assets URLs
config.assets.digest = true

# Defaults to Rails.root.join("public/assets")
# config.assets.manifest = YOUR_PATH

# Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
# config.assets.precompile += %w( search.js )

# Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
# config.force_ssl = true

4.5 config/environments/test.rb

You can help test performance with these additions to your test environment:

# Configure static asset server for tests with Cache-Control for performance
config.serve_static_assets = true
config.static_cache_control = "public, max-age=3600"

4.6 config/initializers/wrap_parameters.rb

Add this file with the following contents, if you wish to wrap parameters into a nested hash. This is on by default in new applications.

# Be sure to restart your server when you modify this file.
# This file contains settings for ActionController::ParamsWrapper which
# is enabled by default.

# Enable parameter wrapping for JSON. You can disable this by setting :format to an empty array.
ActiveSupport.on_load(:action_controller) do
  wrap_parameters format: [:json]
end

# Disable root element in JSON by default.
ActiveSupport.on_load(:active_record) do
  self.include_root_in_json = false
end

4.7 config/initializers/session_store.rb

You need to change your session key to something new, or remove all sessions:

# in config/initializers/session_store.rb
AppName::Application.config.session_store :cookie_store, key: 'SOMETHINGNEW'

$ rake db:sessions:clear

4.8 Remove :cache and :concat options in asset helpers references in views

With the Asset Pipeline the :cache and :concat options aren't used anymore, delete these options from your views.

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