While you can use initializers like the other answers, the conventional Rails 4.1+ way is to use the config/secrets.yml. The reason for the Rails team to introduce this is beyond the scope of this answer but the TL;DR is that secret_token.rb conflates configuration and code as well as being a security risk since the token is checked into source control history and the only system that needs to know the production secret token is the production infrastructure.
You should add this file to .gitignore much like you wouldn't add config/database.yml to source control either.
Referencing Heroku's own code for setting up config/database.yml from DATABASE_URL in their Buildpack for Ruby, I ended up forking their repo and modified it to create config/secrets.yml from SECRETS_KEY_BASE environment variable.
Since this feature was introduced in Rails 4.1, I felt it was appropriate to edit ./lib/language_pack/rails41.rb and add this functionality.
The following is the snippet from the modified buildpack I created at my company:
class LanguagePack::Rails41 < LanguagePack::Rails4
# ...
def compile
instrument "rails41.compile" do
super
allow_git do
create_secrets_yml
end
end
end
# ...
# writes ERB based secrets.yml for Rails 4.1+
def create_secrets_yml
instrument 'ruby.create_secrets_yml' do
log("create_secrets_yml") do
return unless File.directory?("config")
topic("Writing config/secrets.yml to read from SECRET_KEY_BASE")
File.open("config/secrets.yml", "w") do |file|
file.puts <<-SECRETS_YML
<%
raise "No RACK_ENV or RAILS_ENV found" unless ENV["RAILS_ENV"] || ENV["RACK_ENV"]
%>
<%= ENV["RAILS_ENV"] || ENV["RACK_ENV"] %>:
secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
SECRETS_YML
end
end
end
end
# ...
end
You can of course extend this code to add other secrets (e.g. third party API keys, etc.) to be read off of your environment variable:
...
<%= ENV["RAILS_ENV"] || ENV["RACK_ENV"] %>:
secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
third_party_api_key: <%= ENV["THIRD_PARTY_API"] %>
This way, you can access this secret in a very standard way:
Rails.application.secrets.third_party_api_key
Before redeploying your app, be sure to set your environment variable first:
Then add your modified buildpack (or you're more than welcome to link to mine) to your Heroku app (see Heroku's documentation) and redeploy your app.
The buildpack will automatically create your config/secrets.yml from your environment variable as part of the dyno build process every time you git push to Heroku.
EDIT: Heroku's own documentation suggests creating config/secrets.yml to read from the environment variable but this implies you should check this file into source control. In my case, this doesn't work well since I have hardcoded secrets for development and testing environments that I'd rather not check in.
secret.ymlorsecrets.yml? – James