Configuration

This page documents where configuration lives in the codebase, not the exhaustive list of options. The exhaustive user-facing list is at guides/source/configuring.md (rendered as https://guides.rubyonrails.org/configuring.html).

Where config.* is defined

Configuration is layered:

1. Rails::Application::Configuration — railties/lib/rails/application/configuration.rb. The top-level container. Has direct attributes (config.eager_load, config.cache_classes, config.middleware, etc.) and one nested namespace per framework component.
2. Per-component config namespaces — config.active_record, config.action_controller, config.active_job, etc. Each is an ActiveSupport::OrderedOptions populated by the component's railtie.
3. Component class attributes — many config flags map onto class attributes via the railtie (e.g., config.action_view.field_error_proc becomes ActionView::Base.field_error_proc).

Reading configuration in framework code

Inside framework code:

ActionView::Base.remove_hidden_field_autocomplete

Reading it through the application config:

Rails.application.config.action_view.remove_hidden_field_autocomplete

Both ultimately point at the same cattr_accessor defined on the framework's base class. The railtie wires them together at boot.

The load_defaults system

Rails::Application::Configuration#load_defaults(version) opts the app into the recommended config for a Rails version. Implementation:

• railties/lib/rails/application/configuration.rb — the case statement.
• Each new defaults block flips on the recommended flags for that version.

For example, the 8.1 block (excerpt-style):

case target_version.to_s
when "8.1"
  action_view.remove_hidden_field_autocomplete = true
  ...

When you upgrade an app, you bump config.load_defaults 8.1 to get the new defaults. Apps that haven't bumped continue running with the previous behavior.

The configuration-flag pattern

When adding a flag, follow the pattern documented in how-to-contribute/patterns-and-conventions:

1. cattr_accessor :flag_name, default: false on the component's base class.
2. Read the flag at the call site.
3. Wire into load_defaults for new defaults.
4. Document in guides/source/configuring.md.

Notable namespaces

Namespace	File	Notes
config	railties/lib/rails/application/configuration.rb	Top-level
config.middleware	actionpack/lib/action_dispatch/middleware/stack.rb	Insert / delete middlewares
config.generators	railties/lib/rails/generators.rb	Generator defaults
config.active_support	activesupport/lib/active_support/railtie.rb	Logger, time zone, error reporter
config.active_record	activerecord/lib/active_record/railtie.rb	Migration paths, schema format, query log tags, encryption
config.action_controller	actionpack/lib/action_controller/railtie.rb	Forgery protection, parameter wrappers, log subscribers
config.action_dispatch	actionpack/lib/action_dispatch/railtie.rb	Trusted proxies, hosts, session store
config.action_view	actionview/lib/action_view/railtie.rb	Field error proc, prefix partials, automatic ETags
config.action_mailer	actionmailer/lib/action_mailer/railtie.rb	Default URL options, delivery method
config.action_mailbox	actionmailbox/lib/action_mailbox/engine.rb	Ingress provider, incineration interval
config.active_job	activejob/lib/active_job/railtie.rb	Queue adapter, name prefix, default retries
config.action_cable	actioncable/lib/action_cable/engine.rb	Mount path, allowed origins
config.active_storage	activestorage/lib/active_storage/engine.rb	Service, variant processor, allowed file types
config.action_text	actiontext/lib/action_text/engine.rb	Sanitization rules, attachment tags

Environment-specific config

config/environments/{development,test,production}.rb is loaded between before_configuration and before_initialize callbacks (see features/initialization-and-railties). It's plain Ruby evaluated against Rails.application.config, so it can override anything set elsewhere.

Encrypted credentials

config.credentials is backed by an encrypted YAML file (config/credentials.yml.enc) decrypted with a master key. Implementation:

• railties/lib/rails/secrets.rb
• activesupport/lib/active_support/encrypted_configuration.rb
• activesupport/lib/active_support/encrypted_file.rb

Per-environment credentials live under config/credentials/<env>.yml.enc with their own keys.

Inspecting in an app

bin/rails about           # Versions, env, database
bin/rails initializers    # Resolved initializer order
bin/rails middleware      # Final middleware stack
bin/rails routes          # All defined routes
bin/rails db:schema_cache:dump

These commands are implemented under railties/lib/rails/commands/.

Related pages

• packages/railties
• features/initialization-and-railties
• how-to-contribute/patterns-and-conventions