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Configuring Rails Applications

This guide covers the configuration and initialization features available to Rails applications. By referring to this guide, you will be able to:

1 Locations for Initialization Code

Rails offers four standard spots to place initialization code:

  • config/application.rb
  • Environment-specific configuration files
  • Initializers
  • After-initializers

2 Running Code Before Rails

In the rare event that your application needs to run some code before Rails itself is loaded, put it above the call to require 'rails/all' in config/application.rb.

3 Configuring Rails Components

In general, the work of configuring Rails means configuring the components of Rails, as well as configuring Rails itself. The configuration file config/application.rb and environment-specific configuration files (such as config/environments/production.rb) allow you to specify the various settings that you want to pass down to all of the components.

For example, the default config/application.rb file includes this setting:

config.filter_parameters += [:password]

This is a setting for Rails itself. If you want to pass settings to individual Rails components, you can do so via the same config object in config/application.rb:

config.active_record.observers = [:hotel_observer, :review_observer]

Rails will use that particular setting to configure Active Record.

3.1 Rails General Configuration

These configuration methods are to be called on a Rails::Railtie object, such as a subclass of Rails::Engine or Rails::Application.

  • config.after_initialize takes a block which will be run after Rails has finished initializing the application. That includes the initialization of the framework itself, plugins, engines, and all the application’s initializers in config/initializers. Note that this block will be run for rake tasks. Useful for configuring values set up by other initializers:
config.after_initialize do
  ActionView::Base.sanitized_allowed_tags.delete 'div'
  • config.allow_concurrency should be true to allow concurrent (threadsafe) action processing. False by default. You probably don’t want to call this one directly, though, because a series of other adjustments need to be made for threadsafe mode to work properly. Can also be enabled with threadsafe!.
  • config.asset_host sets the host for the assets. Useful when CDNs are used for hosting assets, or when you want to work around the concurrency constraints builtin in browsers using different domain aliases. Shorter version of config.action_controller.asset_host.
  • config.asset_path lets you decorate asset paths. This can be a callable, a string, or be nil which is the default. For example, the normal path for blog.js would be /javascripts/blog.js, let that absolute path be path. If config.asset_path is a callable, Rails calls it when generating asset paths passing path as argument. If config.asset_path is a string, it is expected to be a sprintf format string with a %s where path will get inserted. In either case, Rails outputs the decorated path. Shorter version of config.action_controller.asset_path.
config.asset_path = proc { |path| "/blog/public#{path}" }

The config.asset_path configuration is ignored if the asset pipeline is enabled, which is the default.

  • config.autoload_once_paths accepts an array of paths from which Rails will autoload constants that won’t be wiped per request. Relevant if config.cache_classes is false, which is the case in development mode by default. Otherwise, all autoloading happens only once. All elements of this array must also be in autoload_paths. Default is an empty array.
  • config.autoload_paths accepts an array of paths from which Rails will autoload constants. Default is all directories under app.
  • config.cache_classes controls whether or not application classes and modules should be reloaded on each request. Defaults to false in development mode, and true in test and production modes. Can also be enabled with threadsafe!.
  • config.action_view.cache_template_loading controls whether or not templates should be reloaded on each request. Defaults to whatever is set for config.cache_classes.
  • config.cache_store configures which cache store to use for Rails caching. Options include one of the symbols :memory_store, :file_store, :mem_cache_store, or an object that implements the cache API. Defaults to :file_store if the directory tmp/cache exists, and to :memory_store otherwise.
  • config.colorize_logging specifies whether or not to use ANSI color codes when logging information. Defaults to true.
  • config.consider_all_requests_local is a flag. If true then any error will cause detailed debugging information to be dumped in the HTTP response, and the Rails::Info controller will show the application runtime context in /rails/info/properties. True by default in development and test environments, and false in production mode. For finer-grained control, set this to false and implement local_request? in controllers to specify which requests should provide debugging information on errors.
  • config.dependency_loading is a flag that allows you to disable constant autoloading setting it to false. It only has effect if config.cache_classes is true, which it is by default in production mode. This flag is set to false by config.threadsafe!.
  • config.eager_load_paths accepts an array of paths from which Rails will eager load on boot if cache classes is enabled. Defaults to every folder in the app directory of the application.
  • config.encoding sets up the application-wide encoding. Defaults to UTF-8.
  • config.exceptions_app sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to
  • config.file_watcher the class used to detect file updates in the filesystem when config.reload_classes_only_on_change is true. Must conform to ActiveSupport::FileUpdateChecker API.
  • config.filter_parameters used for filtering out the parameters that you don’t want shown in the logs, such as passwords or credit card numbers.
  • config.force_ssl forces all requests to be under HTTPS protocol by using Rack::SSL middleware.
  • config.log_level defines the verbosity of the Rails logger. This option defaults to :debug for all modes except production, where it defaults to :info.
  • config.log_tags accepts a list of methods that respond to request object. 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.
  • config.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class. Defaults to an instance of ActiveSupport::BufferedLogger, with auto flushing off in production mode.
  • config.middleware allows you to configure the application’s middleware. This is covered in depth in the Configuring Middleware section below.
  • config.plugins accepts the list of plugins to load. The default is nil in which case all plugins will be loaded. If this is set to [], no plugins will be loaded. Otherwise, plugins will be loaded in the order specified. This option lets you enforce some particular loading order, useful when dependencies between plugins require it. For that use case, put first the plugins you want to be loaded in a certain order, and then the special symbol :all to have the rest loaded without the need to specify them.
  • config.preload_frameworks enables or disables preloading all frameworks at startup. Enabled by config.threadsafe!. Defaults to nil, so is disabled.
  • config.reload_classes_only_on_change enables or disables reloading of classes only when tracked files change. By default tracks everything on autoload paths and is set to true. If config.cache_classes is true, this option is ignored.
  • config.reload_plugins enables or disables plugin reloading. Defaults to false.
  • config.secret_token used for specifying a key which allows sessions for the application to be verified against a known secure key to prevent tampering. Applications get config.secret_token initialized to a random key in config/initializers/secret_token.rb.
  • config.serve_static_assets configures Rails itself to serve static assets. Defaults to true, but in the production environment is turned off as the server software (e.g. Nginx or Apache) used to run the application should serve static assets instead. Unlike the default setting set this to true when running (absolutely not recommended!) or testing your app in production mode using WEBrick. Otherwise you won´t be able use page caching and requests for files that exist regularly under the public directory will anyway hit your Rails app.
  • config.session_store is usually set up in config/initializers/session_store.rb and specifies what class to use to store the session. Possible values are :cookie_store which is the default, :mem_cache_store, and :disabled. The last one tells Rails not to deal with sessions. Custom session stores can also be specified:
config.session_store :my_custom_store

This custom store must be defined as ActionDispatch::Session::MyCustomStore. In addition to symbols, they can also be objects implementing a certain API, like ActiveRecord::SessionStore, in which case no special namespace is required.

  • config.threadsafe! enables allow_concurrency, cache_classes, dependency_loading and preload_frameworks to make the application threadsafe.

Threadsafe operation is incompatible with the normal workings of development mode Rails. In particular, automatic dependency loading and class reloading are automatically disabled when you call config.threadsafe!.

  • config.time_zone sets the default time zone for the application and enables time zone awareness for Active Record.
  • config.whiny_nils enables or disables warnings when a certain set of methods are invoked on nil and it does not respond to them. Defaults to true in development and test environments.

3.2 Configuring Assets

Rails 3.1, by default, is set up to use the sprockets gem to manage assets within an application. This gem concatenates and compresses assets in order to make serving them much less painful.

  • config.assets.enabled a flag that controls whether the asset pipeline is enabled. It is explicitly initialized in config/application.rb.
  • config.assets.compress a flag that enables the compression of compiled assets. It is explicitly set to true in config/production.rb.
  • config.assets.css_compressor defines the CSS compressor to use. It is set by default by sass-rails. The unique alternative value at the moment is :yui, which uses the yui-compressor gem.
  • config.assets.js_compressor defines the JavaScript compressor to use. Possible values are :closure, :uglifier and :yui which require the use of the closure-compiler, uglifier or yui-compressor gems respectively.
  • config.assets.paths contains the paths which are used to look for assets. Appending paths to this configuration option will cause those paths to be used in the search for assets.
  • config.assets.precompile allows you to specify additional assets (other than application.css and application.js) which are to be precompiled when rake assets:precompile is run.
  • config.assets.prefix defines the prefix where assets are served from. Defaults to /assets.
  • config.assets.digest enables the use of MD5 fingerprints in asset names. Set to true by default in production.rb.
  • config.assets.debug disables the concatenation and compression of assets. Set to false by default in development.rb.
  • config.assets.manifest defines the full path to be used for the asset precompiler’s manifest file. Defaults to using config.assets.prefix.
  • config.assets.cache_store defines the cache store that Sprockets will use. The default is the Rails file store.
  • config.assets.version is an option string that is used in MD5 hash generation. This can be changed to force all files to be recompiled.
  • config.assets.compile is a boolean that can be used to turn on live Sprockets compilation in production.
  • config.assets.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class. Defaults to the same configured at config.logger. Setting config.assets.logger to false will turn off served assets logging.

3.3 Configuring Generators

Rails 3 allows you to alter what generators are used with the config.generators method. This method takes a block:

config.generators do |g|
  g.orm :active_record
  g.test_framework :test_unit

The full set of methods that can be used in this block are as follows:

  • assets allows to create assets on generating a scaffold. Defaults to true.
  • force_plural allows pluralized model names. Defaults to false.
  • helper defines whether or not to generate helpers. Defaults to true.
  • integration_tool defines which integration tool to use. Defaults to nil.
  • javascripts turns on the hook for javascripts in generators. Used in Rails for when the scaffold generator is ran. Defaults to true.
  • javascript_engine configures the engine to be used (for eg. coffee) when generating assets. Defaults to nil.
  • orm defines which orm to use. Defaults to false and will use Active Record by default.
  • performance_tool defines which performance tool to use. Defaults to nil.
  • resource_controller defines which generator to use for generating a controller when using rails generate resource. Defaults to :controller.
  • scaffold_controller different from resource_controller, defines which generator to use for generating a scaffolded controller when using rails generate scaffold. Defaults to :scaffold_controller.
  • stylesheets turns on the hook for stylesheets in generators. Used in Rails for when the scaffold generator is ran, but this hook can be used in other generates as well. Defaults to true.
  • stylesheet_engine configures the stylesheet engine (for eg. sass) to be used when generating assets. Defaults to :css.
  • test_framework defines which test framework to use. Defaults to false and will use Test::Unit by default.
  • template_engine defines which template engine to use, such as ERB or Haml. Defaults to :erb.

3.4 Configuring Middleware

Every Rails application comes with a standard set of middleware which it uses in this order in the development environment:

  • Rack::SSL forces every request to be under HTTPS protocol. Will be available if config.force_ssl is set to true. Options passed to this can be configured by using config.ssl_options.
  • ActionDispatch::Static is used to serve static assets. Disabled if config.serve_static_assets is true.
  • Rack::Lock wraps the app in mutex so it can only be called by a single thread at a time. Only enabled if config.action_controller.allow_concurrency is set to false, which it is by default.
  • ActiveSupport::Cache::Strategy::LocalCache serves as a basic memory backed cache. This cache is not thread safe and is intended only for serving as a temporary memory cache for a single thread.
  • Rack::Runtime sets an X-Runtime header, containing the time (in seconds) taken to execute the request.
  • Rails::Rack::Logger notifies the logs that the request has began. After request is complete, flushes all the logs.
  • ActionDispatch::ShowExceptions rescues any exception returned by the application and renders nice exception pages if the request is local or if config.consider_all_requests_local is set to true. If config.action_dispatch.show_exceptions is set to false, exceptions will be raised regardless.
  • ActionDispatch::RequestId makes a unique X-Request-Id header available to the response and enables the ActionDispatch::Request#uuid method.
  • ActionDispatch::RemoteIp checks for IP spoofing attacks. Configurable with the config.action_dispatch.ip_spoofing_check and config.action_dispatch.trusted_proxies settings.
  • Rack::Sendfile intercepts responses whose body is being served from a file and replaces it with a server specific X-Sendfile header. Configurable with config.action_dispatch.x_sendfile_header.
  • ActionDispatch::Callbacks runs the prepare callbacks before serving the request.
  • ActiveRecord::ConnectionAdapters::ConnectionManagement cleans active connections after each request, unless the rack.test key in the request environment is set to true.
  • ActiveRecord::QueryCache caches all SELECT queries generated in a request. If any INSERT or UPDATE takes place then the cache is cleaned.
  • ActionDispatch::Cookies sets cookies for the request.
  • ActionDispatch::Session::CookieStore is responsible for storing the session in cookies. An alternate middleware can be used for this by changing the config.action_controller.session_store to an alternate value. Additionally, options passed to this can be configured by using config.action_controller.session_options.
  • ActionDispatch::Flash sets up the flash keys. Only available if config.action_controller.session_store is set to a value.
  • ActionDispatch::ParamsParser parses out parameters from the request into params.
  • Rack::MethodOverride allows the method to be overridden if params[:_method] is set. This is the middleware which supports the PUT and DELETE HTTP method types.
  • ActionDispatch::Head converts HEAD requests to GET requests and serves them as so.
  • ActionDispatch::BestStandardsSupport enables “best standards support” so that IE8 renders some elements correctly.

Besides these usual middleware, you can add your own by using the config.middleware.use method:

config.middleware.use Magical::Unicorns

This will put the Magical::Unicorns middleware on the end of the stack. You can use insert_before if you wish to add a middleware before another.

config.middleware.insert_before ActionDispatch::Head, Magical::Unicorns

There’s also insert_after which will insert a middleware after another:

config.middleware.insert_after ActionDispatch::Head, Magical::Unicorns

Middlewares can also be completely swapped out and replaced with others:

config.middleware.swap ActionDispatch::BestStandardsSupport, Magical::Unicorns

They can also be removed from the stack completely:

config.middleware.delete ActionDispatch::BestStandardsSupport

3.5 Configuring i18n

  • config.i18n.default_locale sets the default locale of an application used for i18n. Defaults to :en.
  • config.i18n.load_path sets the path Rails uses to look for locale files. Defaults to config/locales/*.{yml,rb}.

3.6 Configuring Active Record

config.active_record includes a variety of configuration options:

  • config.active_record.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then passed on to any new database connections made. You can retrieve this logger by calling logger on either an Active Record model class or an Active Record model instance. Set to nil to disable logging.
  • config.active_record.primary_key_prefix_type lets you adjust the naming for primary key columns. By default, Rails assumes that primary key columns are named id (and this configuration option doesn’t need to be set.) There are two other choices:
    • :table_name would make the primary key for the Customer class customerid
    • :table_name_with_underscore would make the primary key for the Customer class customer_id
  • config.active_record.table_name_prefix lets you set a global string to be prepended to table names. If you set this to northwest_, then the Customer class will look for northwest_customers as its table. The default is an empty string.
  • config.active_record.table_name_suffix lets you set a global string to be appended to table names. If you set this to _northwest, then the Customer class will look for customers_northwest as its table. The default is an empty string.
  • config.active_record.pluralize_table_names specifies whether Rails will look for singular or plural table names in the database. If set to true (the default), then the Customer class will use the customers table. If set to false, then the Customer class will use the customer table.
  • config.active_record.default_timezone determines whether to use Time.local (if set to :local) or Time.utc (if set to :utc) when pulling dates and times from the database. The default is :utc for Rails, although Active Record defaults to :local when used outside of Rails.
  • config.active_record.schema_format controls the format for dumping the database schema to a file. The options are :ruby (the default) for a database-independent version that depends on migrations, or :sql for a set of (potentially database-dependent) SQL statements.
  • config.active_record.timestamped_migrations controls whether migrations are numbered with serial integers or with timestamps. The default is true, to use timestamps, which are preferred if there are multiple developers working on the same application.
  • config.active_record.lock_optimistically controls whether Active Record will use optimistic locking and is true by default.
  • config.active_record.whitelist_attributes will create an empty whitelist of attributes available for mass-assignment security for all models in your app.
  • config.active_record.identity_map controls whether the identity map is enabled, and is false by default.
  • config.active_record.auto_explain_threshold_in_seconds configures the threshold for automatic EXPLAINs (nil disables this feature). Queries exceeding the threshold get their query plan logged. Default is 0.5 in development mode.

The MySQL adapter adds one additional configuration option:

  • ActiveRecord::ConnectionAdapters::MysqlAdapter.emulate_booleans controls whether Active Record will consider all tinyint(1) columns in a MySQL database to be booleans and is true by default.

The schema dumper adds one additional configuration option:

  • ActiveRecord::SchemaDumper.ignore_tables accepts an array of tables that should not be included in any generated schema file. This setting is ignored unless config.active_record.schema_format == :ruby.

3.7 Configuring Action Controller

config.action_controller includes a number of configuration settings:

  • config.action_controller.asset_host sets the host for the assets. Useful when CDNs are used for hosting assets rather than the application server itself.
  • config.action_controller.asset_path takes a block which configures where assets can be found. Shorter version of config.action_controller.asset_path.
  • config.action_controller.page_cache_directory should be the document root for the web server and is set using Base.page_cache_directory = “/document/root”. For Rails, this directory has already been set to Rails.public_path (which is usually set to Rails.root + “/public”). Changing this setting can be useful to avoid naming conflicts with files in public/, but doing so will likely require configuring your web server to look in the new location for cached files.
  • config.action_controller.page_cache_extension configures the extension used for cached pages saved to page_cache_directory. Defaults to .html.
  • config.action_controller.perform_caching configures whether the application should perform caching or not. Set to false in development mode, true in production.
  • config.action_controller.default_charset specifies the default character set for all renders. The default is “utf-8”.
  • config.action_controller.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Controller. Set to nil to disable logging.
  • config.action_controller.request_forgery_protection_token sets the token parameter name for RequestForgery. Calling protect_from_forgery sets it to :authenticity_token by default.
  • config.action_controller.allow_forgery_protection enables or disables CSRF protection. By default this is false in test mode and true in all other modes.
  • config.action_controller.relative_url_root can be used to tell Rails that you are deploying to a subdirectory. The default is ENV['RAILS_RELATIVE_URL_ROOT'].

The caching code adds two additional settings:

  • ActionController::Base.page_cache_directory sets the directory where Rails will create cached pages for your web server. The default is Rails.public_path (which is usually set to Rails.root + “/public”).
  • ActionController::Base.page_cache_extension sets the extension to be used when generating pages for the cache (this is ignored if the incoming request already has an extension). The default is .html.

The Active Record session store can also be configured:

  • ActiveRecord::SessionStore::Session.table_name sets the name of the table used to store sessions. Defaults to sessions.
  • ActiveRecord::SessionStore::Session.primary_key sets the name of the ID column used in the sessions table. Defaults to session_id.
  • ActiveRecord::SessionStore::Session.data_column_name sets the name of the column which stores marshaled session data. Defaults to data.

3.8 Configuring Action Dispatch

  • config.action_dispatch.session_store sets the name of the store for session data. The default is :cookie_store; other valid options include :active_record_store, :mem_cache_store or the name of your own custom class.
  • config.action_dispatch.tld_length sets the TLD (top-level domain) length for the application. Defaults to 1.
  • ActionDispatch::Callbacks.before takes a block of code to run before the request.
  • ActionDispatch::Callbacks.to_prepare takes a block to run after ActionDispatch::Callbacks.before, but before the request. Runs for every request in development mode, but only once for production or environments with cache_classes set to true.
  • ActionDispatch::Callbacks.after takes a block of code to run after the request.

3.9 Configuring Action View

There are only a few configuration options for Action View, starting with four on ActionView::Base:

  • config.action_view.field_error_proc provides an HTML generator for displaying errors that come from Active Record. The default is { |html_tag, instance| %Q(<div class="field_with_errors">#{html_tag}</div>).html_safe }
  • config.action_view.default_form_builder tells Rails which form builder to use by default. The default is ActionView::Helpers::FormBuilder.
  • config.action_view.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Mailer. Set to nil to disable logging.
  • config.action_view.erb_trim_mode gives the trim mode to be used by ERB. It defaults to '-'. See the ERB documentation for more information.
  • config.action_view.javascript_expansions is a hash containing expansions that can be used for the JavaScript include tag. By default, this is defined as:
config.action_view.javascript_expansions = { :defaults => %w(jquery jquery_ujs) }

However, you may add to this by defining others:

config.action_view.javascript_expansions[:prototype] = ['prototype', 'effects', 'dragdrop', 'controls']

And can reference in the view with the following code:

<%= javascript_include_tag :prototype %>
  • config.action_view.stylesheet_expansions works in much the same way as javascript_expansions, but has no default key. Keys defined for this hash can be referenced in the view like such:
<%= stylesheet_link_tag :special %>
  • config.action_view.cache_asset_ids With the cache enabled, the asset tag helper methods will make fewer expensive file system calls (the default implementation checks the file system timestamp). However this prevents you from modifying any asset files while the server is running.

3.10 Configuring Action Mailer

There are a number of settings available on config.action_mailer:

  • config.action_mailer.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Action Mailer. Set to nil to disable logging.
  • config.action_mailer.smtp_settings allows detailed configuration for the :smtp delivery method. It accepts a hash of options, which can include any of these options:
    • :address – Allows you to use a remote mail server. Just change it from its default “localhost” setting.
    • :port – On the off chance that your mail server doesn’t run on port 25, you can change it.
    • :domain – If you need to specify a HELO domain, you can do it here.
    • :user_name – If your mail server requires authentication, set the username in this setting.
    • :password – If your mail server requires authentication, set the password in this setting.
    • :authentication – If your mail server requires authentication, you need to specify the authentication type here. This is a symbol and one of :plain, :login, :cram_md5.
  • config.action_mailer.sendmail_settings allows detailed configuration for the sendmail delivery method. It accepts a hash of options, which can include any of these options:
    • :location – The location of the sendmail executable. Defaults to /usr/sbin/sendmail.
    • :arguments – The command line arguments. Defaults to -i -t.
  • config.action_mailer.raise_delivery_errors specifies whether to raise an error if email delivery cannot be completed. It defaults to true.
  • config.action_mailer.delivery_method defines the delivery method. The allowed values are :smtp (default), :sendmail, and :test.
  • config.action_mailer.perform_deliveries specifies whether mail will actually be delivered and is true by default. It can be convenient to set it to false for testing.
  • config.action_mailer.default configures Action Mailer defaults. These default to:
    :mime_version => "1.0",
    :charset      => "UTF-8",
    :content_type => "text/plain",
    :parts_order  => [ "text/plain", "text/enriched", "text/html" ]
  • config.action_mailer.observers registers observers which will be notified when mail is delivered.
    config.action_mailer.observers = ["MailObserver"]
  • config.action_mailer.interceptors registers interceptors which will be called before mail is sent.
    config.action_mailer.interceptors = ["MailInterceptor"]

3.11 Configuring Active Resource

There is a single configuration setting available on config.active_resource:

  • config.active_resource.logger accepts a logger conforming to the interface of Log4r or the default Ruby Logger class, which is then used to log information from Active Resource. Set to nil to disable logging.

3.12 Configuring Active Support

There are a few configuration options available in Active Support:

  • config.active_support.bare enables or disables the loading of active_support/all when booting Rails. Defaults to nil, which means active_support/all is loaded.
  • config.active_support.escape_html_entities_in_json enables or disables the escaping of HTML entities in JSON serialization. Defaults to true.
  • config.active_support.use_standard_json_time_format enables or disables serializing dates to ISO 8601 format. Defaults to false.
  • ActiveSupport::BufferedLogger.silencer is set to false to disable the ability to silence logging in a block. The default is true.
  • ActiveSupport::Cache::Store.logger specifies the logger to use within cache store operations.
  • ActiveSupport::Deprecation.behavior alternative setter to config.active_support.deprecation which configures the behavior of deprecation warnings for Rails.
  • ActiveSupport::Deprecation.silence takes a block in which all deprecation warnings are silenced.
  • ActiveSupport::Deprecation.silenced sets whether or not to display deprecation warnings.
  • ActiveSupport::Logger.silencer is set to false to disable the ability to silence logging in a block. The default is true.

4 Rails Environment Settings

Some parts of Rails can also be configured externally by supplying environment variables. The following environment variables are recognized by various parts of Rails:

  • ENV["RAILS_ENV"] defines the Rails environment (production, development, test, and so on) that Rails will run under.
  • ENV["RAILS_RELATIVE_URL_ROOT"] is used by the routing code to recognize URLs when you deploy your application to a subdirectory.
  • ENV["RAILS_ASSET_ID"] will override the default cache-busting timestamps that Rails generates for downloadable assets.
  • ENV["RAILS_CACHE_ID"] and ENV["RAILS_APP_VERSION"] are used to generate expanded cache keys in Rails’ caching code. This allows you to have multiple separate caches from the same application.

5 Using Initializer Files

After loading the framework and any gems and plugins in your application, Rails turns to loading initializers. An initializer is any Ruby file stored under config/initializers in your application. You can use initializers to hold configuration settings that should be made after all of the frameworks, plugins and gems are loaded, such as options to configure settings for these parts.

You can use subfolders to organize your initializers if you like, because Rails will look into the whole file hierarchy from the initializers folder on down.

If you have any ordering dependency in your initializers, you can control the load order by naming. For example, 01_critical.rb will be loaded before 02_normal.rb.

6 Initialization events

Rails has 5 initialization events which can be hooked into (listed in the order that they are ran):

  • before_configuration: This is run as soon as the application constant inherits from Rails::Application. The config calls are evaluated before this happens.
  • before_initialize: This is run directly before the initialization process of the application occurs with the :bootstrap_hook initializer near the beginning of the Rails initialization process.
  • to_prepare: Run after the initializers are ran for all Railties (including the application itself), but before eager loading and the middleware stack is built. More importantly, will run upon every request in development, but only once (during boot-up) in production and test.
  • before_eager_load: This is run directly before eager loading occurs, which is the default behaviour for the production environment and not for the development environment.
  • after_initialize: Run directly after the initialization of the application, but before the application initializers are run.

To define an event for these hooks, use the block syntax within a Rails::Aplication, Rails::Railtie or Rails::Engine subclass:

module YourApp
  class Application < Rails::Application
    config.before_initialize do
      # initialization code goes here

Alternatively, you can also do it through the config method on the Rails.application object:

Rails.application.config.before_initialize do
  # initialization code goes here

Some parts of your application, notably observers and routing, are not yet set up at the point where the after_initialize block is called.

6.1 Rails::Railtie#initializer

Rails has several initializers that run on startup that are all defined by using the initializer method from Rails::Railtie. Here’s an example of the initialize_whiny_nils initializer from Active Support:

initializer "active_support.initialize_whiny_nils" do |app|
  require 'active_support/whiny_nil' if app.config.whiny_nils

The initializer method takes three arguments with the first being the name for the initializer and the second being an options hash (not shown here) and the third being a block. The :before key in the options hash can be specified to specify which initializer this new initializer must run before, and the :after key will specify which initializer to run this initializer after.

Initializers defined using the initializer method will be ran in the order they are defined in, with the exception of ones that use the :before or :after methods.

You may put your initializer before or after any other initializer in the chain, as long as it is logical. Say you have 4 initializers called “one” through “four” (defined in that order) and you define “four” to go before “four” but after “three”, that just isn’t logical and Rails will not be able to determine your initializer order.

The block argument of the initializer method is the instance of the application itself, and so we can access the configuration on it by using the config method as done in the example.

Because Rails::Application inherits from Rails::Railtie (indirectly), you can use the initializer method in config/application.rb to define initializers for the application.

6.2 Initializers

Below is a comprehensive list of all the initializers found in Rails in the order that they are defined (and therefore run in, unless otherwise stated).

load_environment_hook Serves as a placeholder so that :load_environment_config can be defined to run before it.

load_active_support Requires active_support/dependencies which sets up the basis for Active Support. Optionally requires active_support/all if config.active_support.bare is un-truthful, which is the default.

preload_frameworks Loads all autoload dependencies of Rails automatically if config.preload_frameworks is true or “truthful”. By default this configuration option is disabled. In Rails, when internal classes are referenced for the first time they are autoloaded. :preload_frameworks loads all of this at once on initialization.

initialize_logger Initializes the logger (an ActiveSupport::BufferedLogger object) for the application and makes it accessible at Rails.logger, provided that no initializer inserted before this point has defined Rails.logger.

initialize_cache If RAILS_CACHE isn’t set yet, initializes the cache by referencing the value in config.cache_store and stores the outcome as RAILS_CACHE. If this object responds to the middleware method, its middleware is inserted before Rack::Runtime in the middleware stack.

set_clear_dependencies_hook Provides a hook for active_record.set_dispatch_hooks to use, which will run before this initializer. This initializer — which runs only if cache_classes is set to false — uses ActionDispatch::Callbacks.after to remove the constants which have been referenced during the request from the object space so that they will be reloaded during the following request.

initialize_dependency_mechanism If config.cache_classes is true, configures ActiveSupport::Dependencies.mechanism to require dependencies rather than load them.

bootstrap_hook Runs all configured before_initialize blocks.

i18n.callbacks In the development environment, sets up a to_prepare callback which will call I18n.reload! if any of the locales have changed since the last request. In production mode this callback will only run on the first request.

active_support.initialize_whiny_nils Requires active_support/whiny_nil if config.whiny_nils is true. This file will output errors such as:

Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id


You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each

active_support.deprecation_behavior Sets up deprecation reporting for environments, defaulting to :log for development, :notify for production and :stderr for test. If a value isn’t set for config.active_support.deprecation then this initializer will prompt the user to configure this line in the current environment’s config/environments file. Can be set to an array of values.

active_support.initialize_time_zone Sets the default time zone for the application based on the config.time_zone setting, which defaults to “UTC”.

action_dispatch.configure Configures the ActionDispatch::Http::URL.tld_length to be set to the value of config.action_dispatch.tld_length.

action_view.cache_asset_ids Sets ActionView::Helpers::AssetTagHelper::AssetPaths.cache_asset_ids to false when Active Support loads, but only if config.cache_classes is too.

action_view.javascript_expansions Registers the expansions set up by config.action_view.javascript_expansions and config.action_view.stylesheet_expansions to be recognized by Action View and therefore usable in the views.

action_view.set_configs Sets up Action View by using the settings in config.action_view by send’ing the method names as setters to ActionView::Base and passing the values through.

action_controller.logger Sets ActionController::Base.logger — if it’s not already set — to Rails.logger.

action_controller.initialize_framework_caches Sets ActionController::Base.cache_store — if it’s not already set — to RAILS_CACHE.

action_controller.set_configs Sets up Action Controller by using the settings in config.action_controller by send’ing the method names as setters to ActionController::Base and passing the values through.

action_controller.compile_config_methods Initializes methods for the config settings specified so that they are quicker to access.

active_record.initialize_timezone Sets ActiveRecord::Base.time_zone_aware_attributes to true, as well as setting ActiveRecord::Base.default_timezone to UTC. When attributes are read from the database, they will be converted into the time zone specified by

active_record.logger Sets ActiveRecord::Base.logger — if it’s not already set — to Rails.logger.

active_record.set_configs Sets up Active Record by using the settings in config.active_record by send’ing the method names as setters to ActiveRecord::Base and passing the values through.

active_record.initialize_database Loads the database configuration (by default) from config/database.yml and establishes a connection for the current environment.

active_record.log_runtime Includes ActiveRecord::Railties::ControllerRuntime which is responsible for reporting the time taken by Active Record calls for the request back to the logger.

active_record.set_dispatch_hooks Resets all reloadable connections to the database if config.cache_classes is set to false.

action_mailer.logger Sets ActionMailer::Base.logger — if it’s not already set — to Rails.logger.

action_mailer.set_configs Sets up Action Mailer by using the settings in config.action_mailer by send’ing the method names as setters to ActionMailer::Base and passing the values through.

action_mailer.compile_config_methods Initializes methods for the config settings specified so that they are quicker to access.

active_resource.set_configs Sets up Active Resource by using the settings in config.active_resource by send’ing the method names as setters to ActiveResource::Base and passing the values through.

set_load_path This initializer runs before bootstrap_hook. Adds the vendor, lib, all directories of app and any paths specified by config.load_paths to $LOAD_PATH.

set_autoload_paths This initializer runs before bootstrap_hook. Adds all sub-directories of app and paths specified by config.autoload_paths to ActiveSupport::Dependencies.autoload_paths.

add_routing_paths Loads (by default) all config/routes.rb files (in the application and railties, including engines) and sets up the routes for the application.

add_locales Adds the files in config/locales (from the application, railties and engines) to I18n.load_path, making available the translations in these files.

add_view_paths Adds the directory app/views from the application, railties and engines to the lookup path for view files for the application.

load_environment_config Loads the config/environments file for the current environment.

append_asset_paths Finds asset paths for the application and all attached railties and keeps a track of the available directories in config.static_asset_paths.

prepend_helpers_path Adds the directory app/helpers from the application, railties and engines to the lookup path for helpers for the application.

load_config_initializers Loads all Ruby files from config/initializers in the application, railties and engines. The files in this directory can be used to hold configuration settings that should be made after all of the frameworks and plugins are loaded.

engines_blank_point Provides a point-in-initialization to hook into if you wish to do anything before engines are loaded. After this point, all railtie and engine initializers are ran.

add_generator_templates Finds templates for generators at lib/templates for the application, railities and engines and adds these to the config.generators.templates setting, which will make the templates available for all generators to reference.

ensure_autoload_once_paths_as_subset Ensures that the config.autoload_once_paths only contains paths from config.autoload_paths. If it contains extra paths, then an exception will be raised.

add_to_prepare_blocks The block for every config.to_prepare call in the application, a railtie or engine is added to the to_prepare callbacks for Action Dispatch which will be ran per request in development, or before the first request in production.

add_builtin_route If the application is running under the development environment then this will append the route for rails/info/properties to the application routes. This route provides the detailed information such as Rails and Ruby version for public/index.html in a default Rails application.

build_middleware_stack Builds the middleware stack for the application, returning an object which has a call method which takes a Rack environment object for the request.

eager_load! If config.cache_classes is true, runs the config.before_eager_load hooks and then calls eager_load! which will load all the Ruby files from config.eager_load_paths.

finisher_hook Provides a hook for after the initialization of process of the application is complete, as well as running all the config.after_initialize blocks for the application, railties and engines.

set_routes_reloader Configures Action Dispatch to reload the routes file using ActionDispatch::Callbacks.to_prepare.

disable_dependency_loading Disables the automatic dependency loading if the config.cache_classes is set to true and config.dependency_loading is set to false.


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

If you see any typos or factual errors you are confident to patch, please clone docrails and push the change yourself. That branch of Rails has public write access. Commits are still reviewed, but that happens after you've submitted your contribution. docrails is cross-merged with master periodically.

You may also find incomplete content, or stuff that is not up to date. Please do add any missing documentation for master. 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 in the rubyonrails-docs mailing list.