Configuration files for rails can be found in config/environments/. By default rails has 3 environments, development, production and test. By editing each file you are editing the configuration for that environment only.

Rails also has a configuration file in config/application.rb. This is a common configuration file as any settings defined here are overwritten by the config specified in each environment.

You add or modify configuration options within the Rails.application.configure do block and configuration options start with config.

Database configuration of a rails project lies in a file config/database.yml. If you create a project using rails new command and don't specify a database engine to be used then rails uses sqlite as the default database. A typical database.yml file with default configuration will look similar to following.

# SQLite version 3.x
#   gem install sqlite3
#
#   Ensure the SQLite 3 gem is defined in your Gemfile
#   gem 'sqlite3'
#
default: &default
  adapter: sqlite3
  pool: 5
  timeout: 5000

development:
  <<: *default
  database: db/development.sqlite3

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: db/test.sqlite3

production:
  <<: *default
  database: db/production.sqlite3

If you want to change the default database while creating a new project you can specify database: rails new hello_world --database=mysql

The following configuration options should be called on a Rails::Railtie object

• config.after_initialize: Takes a block which will be run after rails has initialized the application.
• config.asset_host: This sets the host for the assets. This is useful when using a Content Delivery Network. This is shorthand for config.action_controller.asset_host
• config.autoload_once_paths: This option accepts an array of paths where Rails autoloads constants. The default value is an empty array
• config.autoload_paths: This accepts an array of paths where Rails autoloads constants. It defaults to all directories under app
• config.cache_classes: Determines if classes and modules should be reloaded on each request. In development mode, this defaults to false and in the production and test modes it defaults to true
• config.action_view.cache_template_loading: This determines if templates should be reloaded on each request. It defaults to the config.cache_classes setting
• config.beginning_of_week: This sets the default beginning of week. It requires a valid week day symbol (:monday)
• config.cache_store: Choose which cache store to use. Options include :file_store, :memory_store, mem_cache_store or null_store.
• config.colorize_logging: This controls whether logging information is colorized
• config.eager_load: Eager-loads all registered
• config.encoding: Specifies the application encoding. The default value is UTF-8
• config.log_level: Sets the verbosity of the Rails Logger. It defaults to :debug in all environments.
• config.middleware: Use this to configure the application's middleware
• config.time_zone: This sets the application's default time zone.

The following configuration options can be used for configuring assets

• config.assets.enabled: Determines whether the asset pipeline is enabled. This defaults to true
• config.assets.raise_runtime_errors: This enables runtime error checking. It's useful for development mode
• config.assets.compress: Lets assets be compressed. In production mode, this defaults to true
• config.assets.js_compressor: Specifies which JS compressor to use. Options include :closure, :uglifier and :yui
• config.assets.paths: Specifies which paths to search for assets.
• config.assets.precompile: Lets you choose additional assets to be precompiled when rake assets:precompile is run
• config.assets.digest: This option allows the use of MD-5 fingerprints in the asset names. It defaults to true in development mode
• config.assets.compile: Toggles live Sprockets compilation in production mode

Rails allows you to configure what generators are used when running rails generate commands. This method, config.generators takes a block

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

Here are some of the options

Option	Description	Default
assets	Creates assets when generating scaffold	true
force_plural	Allows pluralized model names	false
helper	Determines whether to generate helpers	true
integration_tool	Specify integration tool	test_unit
javascript_engine	Configures JS engine	:js
resource_route	Generates resource route	true
stylesheet_engine	Configures stylesheet engine	:cs
scaffold_stylesheet	Creates CSS upon scaffolding	true
test_framework	Specify Test Framework	Minitest
template_engine	Configures template engine	:erb