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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:

  • Adjust the behavior of your Rails applications
  • Add additional code to be run at application start time


Locations for Initialization Code

Rails offers (at least) four good spots to place initialization code:

  • application.rb
  • Environment-specific Configuration Files
  • Initializers
  • After-Initializers

Running Code Before Rails

To run some code before Rails itself is loaded, simply put it above the call to
require ‘rails/all’ in your application.rb.

Configuring Rails Components

In general, the work of configuring Rails means configuring the components of Rails, as well as configuring Rails itself. The 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 Rails 3.0 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:

config.active_record.timestamped_migrations = false

Rails will use that particular setting to configure Active Record.

Rails General Configuration

  • config.after_initialize takes a block which will be ran after Rails has finished initializing. Useful for configuring values set up by other initializers:
config.after_initialize do ActionView::Base.sanitized_allowed_tags.delete ‘div’ end
  • config.allow_concurrency should be set to true to allow concurrent (threadsafe) action processing. Set to 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 rather than the application server itself. Shorter version of config.action_controller.asset_host.
  • config.asset_path takes a block which configures where assets can be found. Shorter version of config.action_controller.asset_path.
config.asset_path = proc { |asset_path| “assets/#{asset_path}” }
  • config.autoload_once_paths accepts an array of paths from which Rails will automatically load from only once. All elements of this array must also be in autoload_paths.
  • config.autoload_paths accepts an array of additional paths to prepend to the load path. By default, all app, lib, vendor and mock paths are included in this list.
  • config.cache_classes controls whether or not application classes should be reloaded on each request. Defaults to true in development, false in test and production. Can also be enabled with threadsafe!.
  • config.cache_store configures which cache store to use for Rails caching. Options include :memory_store, :file_store, :mem_cache_store or the name of your own custom class.
  • config.colorize_logging specifies whether or not to use ANSI color codes when logging information. Defaults to true.
  • config.consider_all_requests_local is generally set to true during development and false during production; if it is set to true, then any error will cause detailed debugging information to be dumped in the HTTP response. 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.controller_paths configures where Rails can find controllers for this application.
  • config.dependency_loading enables or disables dependency loading during the request cycle. Setting dependency_loading to true will allow new classes to be loaded during a request and setting it to false will disable this behavior. Can also be enabled with threadsafe!.
  • config.eager_load_paths accepts an array of paths from which Rails will eager load on boot if cache classes is enabled. All elements of this array must also be in load_paths.
  • config.encoding sets up the application-wide encoding. Defaults to UTF-8.
  • 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.helper_paths configures where Rails can find helpers for this application.
  • config.log_level defines the verbosity of the Rails logger. In production mode, this defaults to :info. In development mode, it defaults to :debug.
  • config.log_path overrides the path to the log file to use. Defaults to log/#{environment}.log (e.g. log/development.log or log/production.log).
  • config.logger accepts a logger conforming to the interface of Log4r or the default Ruby 1.8+ Logger class, which is then used to log information from Action Controller. Set to nil to disable logging.
  • 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. If this is set to nil, all plugins will be loaded. If this is set to [], no plugins will be loaded. Otherwise, plugins will be loaded in the order specified.
  • config.preload_frameworks enables or disables preloading all frameworks at startup. Can also be enabled with threadsafe!.
  • config.reload_plugins enables or disables plugin reloading.
  • config.root configures the root path of the application.
  • 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.
  • config.serve_static_assets configures Rails to serve static assets. Defaults to true, but in the production environment is turned off. The server software used to run the application should be used to serve the assets instead.
  • config.threadsafe! enables allow_concurrency, cache_classes, dependency_loading and preload_frameworks to make the application threadsafe.

WARNING: 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 disabled warnings when an methods of nil are invoked. Defaults to false.

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 end

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

  • force_plural allows pluralized model names. Defaults to false.
  • helper defines whether or not to generate helpers. Defaults to true
  • orm defines which orm to use. Defaults to nil, so will use Active Record by default.
  • integration_tool defines which integration tool to use. Defaults to nil
  • 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.
  • test_framework defines which test framework to use. Defaults to nil, so will use Test::Unit by default.
  • template_engine defines which template engine to use, such as ERB or Haml. Defaults to :erb.

Configuring Middleware

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

  • ActionDispatch::Static is used to serve static assets. Disabled if config.serve_static_assets is true.
  • Rack::Lock Will wrap 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 Will notify 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.showexceptions+ is set to false, exceptions will be raised regardless.
  • 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 The Sendfile middleware 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_
  • 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 an 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. If you wish to put this middleware before another use insert_before:

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

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}

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 1.8.x 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 Customers 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 :local.
  • 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 ActiveRecord will use optimistic locking. By default this is true.

The MySQL adapter adds one additional configuration option:

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

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.

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 1.8+ Logger class, which is then used to log information from Action Controller. Set to nil to disable logging.
  • config.action_controller.resource_action_separator gives the token to be used between resources and actions when building or interpreting RESTful URLs. By default, this is “/”.
  • config.action_controller.resource_path_names is a hash of default names for several RESTful actions. By default, the new action is named new and the edit action is named edit.
  • 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.optimise_named_routes turns on some optimizations in generating the routing table. It is set to true by default.
  • config.action_controller.use_accept_header sets the rules for determining the response format. If this is set to true (the default) then respond_to and Request#format will take the Accept header into account. If it is set to false then the request format will be determined solely by examining params[:format]. If there is no format parameter, then the response format will be either HTML or Javascript depending on whether the request is an AJAX request.
  • 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.

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.

Configuring Action View

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

  • config.action_view.debug_rjs specifies whether RJS responses should be wrapped in a try/catch block that alert()s the caught exception (and then re-raises it). The default is false.
  • 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 1.8+ 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.

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 1.8+ 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. By default this is true; 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” ]

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 1.8+ Logger class, which is then used to log information from Active Resource. Set to nil to disable logging.

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::Logger.silencer is set to false to disable the ability to silence logging in a block. The default is true.

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.

Initialization events

Rails has 5 initialization events which can be hooked into (listed in 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.
  • 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.
  • 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 enviroment.
  • after_initialize: Run directly after the initialization of the application, but before the application initializers are run.

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


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 end

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.

WARNING: 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’s argument of the initialize is the instance of the application itself, and so we can access the configuration on it by using the config method as this initializer does.

Initializer files

After loading the framework and any gems and plugins in your application, Rails turns to loading initialization code from config/initializers. 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.

NOTE: 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.

TIP: 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.


  • November 26, 2010: Removed all config settings not available in Rails 3 (Ryan Bigg)
  • August 13, 2009: Updated with config syntax and added general configuration options by “John Pignata”
  • January 3, 2009: First reasonably complete draft by Mike Gunderloy
  • November 5, 2008: Rough outline by Mike Gunderloy
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