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The are multiple ways of configuring your Rails application for different environments (e.g. staging, production, etc). One of the popular ones is through environment variables. For example Heroku uses this type of configuration extensively.

One of the benefits of it is that configuration values are never stored in the source control system, which improves security (for sensitive configuration parameters) and also makes it easier to try different configuration setups w/o changing the sources or re-deploying the application.

On the other hand writing (ENV['PRIMARY_DOMAIN'] || "myapp.com") every time you need your domain string becomes cumbersome pretty fast, not to mention duplication and having the default repeated all over the place.

A competent programmer will of course only do this once, and re-use the value everywhere. Something like this:

PRIMARY_DOMAIN = ENV['PRIMARY_DOMAIN'].presence || 'myapp.com'
S3_BUCKET = ENV['S3_BUCKET'] || raise 'missing S3_BUCKET'
ORDER_EXPIRATION_DAYS = (ENV['ORDER_EXPIRATION_DAYS'].presence || 1).to_i

But it quickly becomes complicated, and again, quite a bit of similarly looking code that begs to be refactored out.

constfig is something I extracted from a couple of my latest projects. It allows you to do just that, have a configuration parameters stored in constants with values coming from environment variables and ability to provide defaults or have required parameters (i.e. fail if missing).

I just released version 0.0.1 of constfig to rubygems. Sources are of course on github.

Installation

Add this line to your application's Gemfile:

gem 'constfig'

And then execute:

$ bundle

Or install it yourself as:

$ gem install constfig

Usage

There is only one function provided by the gem: define_config.

With a default (optional variable)

You can call it with a default, like this:

define_config :DEFAULT_DOMAIN, "astrails.com"

In which case it will first look if ENV['DEFAULT_DOMAIN'] is available, and if not will use the 'astrails.com'. A constant DEFAULT_DOMAIN will be defined.

Without a default (required variable)

Or you can call it without the default:

define_config :DEFAULT_DOMAIN

In which case it will raise exception Constfig::Undefined if ENV['DEFAULT_DOMAIN'] is not available.

Variable type

One last thing. Non-string variables are supported. If you provide a non-string default (boolean, integer, float or symbol), the value that is coming from ENV will be converted to the same type (using to_i, to_f, and to_symbol). For the true/false types "true", "TRUE", and "1" will be treated as true, anything else will be treated as false.

In the case of required variables, you can supply a Class in place of the default, and it will be used for the type conversion. Like this:

define_config :EXPIRATION_DAYS, Fixnum

For boolean variables you can supply either TrueClass, or FalseClass.

Existing constants

This gem will not re-define existing constants, which can be used to define defaults for non-production environments.

Rails on Heroku

There is one caveat with Rails on Heroku. By default Heroku doesn't provide environment variables to your application during the rake assets:precompile stage of slug compilation. If you don't take care of it your application will fail to compile its assets and might fail to work in production. To take care of it you can either use Heroku Labs user-env-compile option, or (and this is what I'd recommend) you can use development defaults during assets:precompile.

For example in Rails you con do this:

if Rails.env.development? || Rails.env.test? || ARGV.join =~ /assets:precompile/
  DEFAULT_DOMAIN = 'myapp.dev'
end

define_config :DEFAULT_DOMAIN

In development and test environments it will use 'myapp.dev' ad PRIMARY_DOMAIN, but in production and staging environment it will fail unless PRIMARY_DOMAIN is provided by environment.

NOTE: make sure those configuration variables are not actually used for asset compilation. If they are, I'd go with user-env-compile.

Managing environment

You can use the dotenv gem to manage your ENV.

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