Custom application configuration variables in Rails 4 and 5

… and check why 5600+ Rails engineers read also this

Custom application configuration variables in Rails 4 and 5

You probably know that you can configure Rails in config/application.rb and config/environments/development.rb etc. But you can also leverage that for configuring your own custom settings for your application.

Single level configuration

# config/environments/development.rb
# or
# config/environments/production.rb

Rails.application.configure do
  config.my_custom_setting = "WOW"
end

# config/application.rb
module ApplicationName
  class Application < Rails::Application
    config.load_defaults 5.1

    config.my_custom_setting = "DEFAULT"
  end
end

As with normal rails configuration, your own settings in config/environments/* take precedence over those specified in config/application.rb.

Nested configuration

If you have multiple settings that you would like to group together, you can prefix them with x.config_name.

# config/environments/development.rb
# or
# config/environments/production.rb

Rails.application.configure do
  config.x.external_api.timeout = 5
end

# config/application.rb
module ApplicationName
  class Application < Rails::Application
    config.load_defaults 5.1

    config.x.external_api.timeout = 30
    config.x.external_api.url = "https://example.org/api/path"
  end
end

or you can just use Hash

# config/environments/development.rb
# or
# config/environments/production.rb

Rails.application.configure do
  config.external_api[:timeout] = 5
end

# config/application.rb
module ApplicationName
  class Application < Rails::Application
    config.load_defaults 5.1

    config.external_api = {
      timeout: 30,
      url: "https://example.org/api/path"
    }
  end
end

Accessing in your model or controller

To read those preference, you can use Rails.application.config or its shorter version Rails.configuration.

class User < ApplicationRecord
  def avatar
    avatar_url || Rails.configuration.default_avatar_url
  end
end

Rails.configuration.x.external_api
# => {:timeout=>5, :url=>"https://example.org/api/path"}

Rails.configuration.x.external_api.timeout
# => 5

Rails.configuration.x.external_api.url
# => "https://example.org/api/path"

Rails.configuration.x.external_api.missing_element
# => nil

It will work anywhere in your application, including models and controllers.

How does the x.my_config.something work under the hood?

What’s the magic that allows you to define the keys in such way?

Rails.configuration.x.class
# => Rails::Application::Configuration::Custom

Let’s see if the setters and getters are implemented using method_missing ?

Rails.configuration.x.method(:method_missing).source_location
 => ["/home/rupert/.rvm/gems/ruby-2.4.1/gems/railties-5.1.4/lib/rails/application/configuration.rb", 200]

Indeed they are. Let’s see the code.

class Custom
  def initialize
    @configurations = Hash.new
  end

  def method_missing(method, *args)
    if method =~ /=$/
      @configurations[$`.to_sym] = args.first
    else
      @configurations.fetch(method) {
        @configurations[method] = ActiveSupport::OrderedOptions.new
      }
    end
  end

  def respond_to_missing?(symbol, *)
    true
  end
end

That’s quite a simple and small implementation. If it recognizes that the method name ends with = it sets the value. If it does not end with =, it sets ActiveSupport::OrderedOptions.new as the value and returns it.

Rails.configuration.x
# => #<Rails::Application::Configuration::Custom:0x000000047a42a8 @configurations={}>

Rails.configuration.x.one
# => {}

Rails.configuration.x.one.class
# => ActiveSupport::OrderedOptions

So the 1st level (x.one, x.one=) is managed by Rails::Application::Configuration::Custom class and its implementation. The 2nd level (x.one.two, x.one.two=) is managed by ActiveSupport::OrderedOptions.

Rails.configuration.x.one.two
# => nil

Rails.configuration.x.one.two = 2
# => 2

Let’s see what’s there.

ActiveSupport::OrderedOptions.new.method(:method_missing).source_location
 => ["/home/rupert/.rvm/gems/ruby-2.4.1/gems/activesupport-5.1.4/lib/active_support/ordered_options.rb", 39]

class OrderedOptions < Hash
  alias_method :_get, :[] # preserve the original #[] method
  protected :_get # make it protected

  def []=(key, value)
    super(key.to_sym, value)
  end

  def [](key)
    super(key.to_sym)
  end

  def method_missing(name, *args)
    name_string = name.to_s
    if name_string.chomp!("=")
      self[name_string] = args.first
    else
      bangs = name_string.chomp!("!")

      if bangs
        fetch(name_string.to_sym).presence || raise(KeyError.new("#{name_string} is blank."))
      else
        self[name_string]
      end
    end
  end

  def respond_to_missing?(name, include_private)
    true
  end
end

And that’s how it works on the 2nd level.

oo = ActiveSupport::OrderedOptions.new
# => {}

oo.foo = 1
# => 1
oo.foo
# => 1
oo.foo!
# => 1
oo.bar
# => nil

oo.bar!
#KeyError: key not found: :bar
#   from (irb):13

Would you like to continue learning more?

If you enjoyed the article, subscribe to our newsletter so that you are always the first one to get the knowledge that you might find useful in your everyday Rails programmer job.

Content is mostly focused on (but not limited to) Ruby, Rails, Web-development and refactoring big, complex Rails applications.

You might also like