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

Rails allows you to use a preinitializer to run code before the framework itself is loaded. If you save code in RAILS_ROOT/config/preinitializer.rb, that code will be the first thing loaded, before any of the framework components (Active Record, Action Pack, and so on.) If you want to change the behavior of one of the classes that is used in the initialization process, you can do so in this file.

In general, the work of configuring Rails means configuring the components of Rails, as well as configuring Rails itself. The environment.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 2.3 environment.rb file includes one setting:

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:

Rails will use that particular setting to configure Active Record.

ActiveRecord::Base includes a variety of configuration options:

logger accepts a logger conforming to the interface of Log4r or the default Ruby 1.8+ Logger class, which is then passed on to any new database connections made. You can retrieve this logger by calling logger on either an ActiveRecord model class or an ActiveRecord model instance. Set to nil to disable logging.

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_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.

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.

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.

colorize_logging (true by default) specifies whether or not to use ANSI color codes when logging information from ActiveRecord.

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.

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.

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.

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 ActiveRecord::Base.schema_format == :ruby.

ActionController::Base includes a number of configuration settings:

asset_host provides a string that is prepended to all of the URL-generating helpers in AssetHelper. This is designed to allow moving all javascript, CSS, and image files to a separate asset host.

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? to specify which requests should provide debugging information on errors.

allow_concurrency should be set to true to allow concurrent (threadsafe) action processing. Set to false by default.

param_parsers provides an array of handlers that can extract information from incoming HTTP requests and add it to the params hash. By default, parsers for multipart forms, URL-encoded forms, XML, and JSON are active.

default_charset specifies the default character set for all renders. The default is "utf-8".

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.

resource_action_separator gives the token to be used between resources and actions when building or interpreting RESTful URLs. By default, this is "/".

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.

request_forgery_protection_token sets the token parameter name for RequestForgery. Calling protect_from_forgery sets it to :authenticity_token by default.

optimise_named_routes turns on some optimizations in generating the routing table. It is set to true by default.

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.

allow_forgery_protection enables or disables CSRF protection. By default this is false in test mode and true in all other modes.

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::Caching::Pages.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::Caching::Pages.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 dispatcher includes one setting:

ActionController::Dispatcher.error_file_path gives the path where Rails will look for error files such as 404.html. The default is Rails.public_path.

The Active Record session store can also be configured:

CGI::Session::ActiveRecordStore::Session.data_column_name sets the name of the column to use to store session data. By default it is data

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

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.

warn_cache_misses tells Rails to display a warning whenever an action results in a cache miss on your view paths. The default is false.

default_form_builder tells Rails which form builder to use by default. The default is ActionView::Helpers::FormBuilder.

The ERB template handler supplies one additional option:

ActionView::TemplateHandlers::ERB.erb_trim_mode gives the trim mode to be used by ERB. It defaults to -. See the ERB documentation for more information.

There are a number of settings available on ActionMailer::Base:

template_root gives the root folder for Action Mailer templates.

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.

smtp_settings allows detailed configuration for the :smtp delivery method. It accepts a hash of options, which can include any of these options:

sendmail_settings allows detailed configuration for the sendmail delivery method. It accepts a hash of options, which can include any of these options:

raise_delivery_errors specifies whether to raise an error if email delivery cannot be completed. It defaults to true.

delivery_method defines the delivery method. The allowed values are :smtp (default), :sendmail, and :test.

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.

default_charset tells Action Mailer which character set to use for the body and for encoding the subject. It defaults to utf-8.

default_content_type specifies the default content type used for the main part of the message. It defaults to "text/plain"

default_mime_version is the default MIME version for the message. It defaults to 1.0.

default_implicit_parts_order - When a message is built implicitly (i.e. multiple parts are assembled from templates which specify the content type in their filenames) this variable controls how the parts are ordered. Defaults to ["text/html", "text/enriched", "text/plain"]. Items that appear first in the array have higher priority in the mail client and appear last in the mime encoded message.

There is a single configuration setting available on ActiveResource::Base:

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.

There are a few configuration options available in Active Support:

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.

Active Model currently has a single configuration setting:

ActiveModel::Errors.default_error_messages is an array containing all of the validation error messages.

After it loads the framework plus any gems and plugins in your application, Rails turns to loading initializers. An initializer is any file of ruby code stored under /config/initializers in your application. You can use initializers to hold configuration settings that should be made after all of the frameworks and plugins are loaded.

After-initializers are run (as you might guess) after any initializers are loaded. You can supply an after_initialize block (or an array of such blocks) by setting up config.after_initialize in any of the Rails configuration files:

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.

ENV[RAILS_GEM_VERSION] defines the version of the Rails gems to use, if RAILS_GEM_VERSION is not defined in your environment.rb file.

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