Sprinkling constants doth maketh a mad-man

You’ve a bunch of configuration constants in your application (email addresses, host addresses, etc) and you’d like a nice, cozy place to keep them, and a method of overriding the defaults for each environment.

There’s a million ways to skin this cat, but here’s the simple approach I took in my latest app.

...and yes, a constant that changes is not a constant, but I’ll pretend you didn’t say that.

1st update: I originally had a typo: the variables should be declared with @@, not @.

2nd update: It’s active_support, not activesupport!

Creatin tha module

Firstly, we’ll set up a new module to store our config, let’s call it MyAppConfig and it can live in RAILS_ROOT/config/my_app_config.rb. In it, we’ll create a standard Ruby module:

module MyApp
  module Config
  end
end

Hookin it in

To allow each environment to customise the config we’ll want to set this up before Rails::Initializer#run is called within environment.rb (see my previous article Environments and the Rails Initialisation Process for more info).

We’ll simply require the file after boot in our environment.rb:

require File.join(File.dirname(__FILE__), 'boot')

require 'my_app_config'

Rails::Initializer.run do |config|
  # ...

Next we’ll add a configurable variable, with a default, to our module:

module MyApp
  module Config
    @@paypal_host = "https://www.sandbox.paypal.com" 
  end
end

This works just fine, apart from the fact you can’t access the instance variable outside of the module:

irb> MyApp::Config.paypal_host
NoMethodError: undefined method `paypal_host' for MyApp::Config:Module

To access and modify the instance variable we need a set of good ‘ol accessors. If this were a class, not a module, we’d sprinkle an attr_accessor and be on our way, but as this is a module we have to define the methods ourself.

module MyApp
  module Config
    @@paypal_host = "https://www.sandbox.paypal.com" 
    def self.paypal_host
      @@paypal_host
    end
    def self.paypal_host=(paypal_host)
      @@paypal_host = paypal_host
    end
  end
end

irb> MyApp::Config.paypal_host
=> "https://www.sandbox.paypal.com"

Sprinklin the sexy

Ok, so far it’s not looking too pretty is it? Don’t worry… good ‘ol ActiveSupport ships with a nice little helper, mattr_accessor, which provides exactly the same functionality as Ruby’s attr_accessor but for modules. Unfortunately the author chose for it a life of seclusion, with a tattoo of :nodoc:, but that’s never stopped us brave venturers before.

ActiveSupport isn’t loaded until the Rails::Initializer#run method is executed, so we’ll also need to require 'activesupport' before we can use mattr_accessor:

require 'active_support'

module MyApp
   module Config
    mattr_accessor :paypal_host
    @@paypal_host = "https://www.sandbox.paypal.com" 
  end
end

So now we have our environment.rb requiring this module and we can access it anywhere in our Rails application with a simple MyApp::Config. paypal_host.

Overidin the defaults

To override config vars in different environments you can simply call its mattr_writer. For example, we’ll want to use the live paypal server in production, so we add the following line to the bottom of RAILS_ROOT/config/environments/production.rb:

MyApp::Config.paypal_host = "https://www.paypal.com"

and voila! That’s all there is to it.

So you wanna get funky

...and here’s some exercises for the reader (all of which are a purely academic pursuit I’d most probably not bother using):

• Extending Module with mattr_accessor_with_default
• Extending Rails::Config to provide a my_app method, so instead of MyApp::Config.paypal_host in your environments you could just use my_app.paypal_host
• Implement MyApp.config(&block) for block-style configuration