It's important to fetch Ruby environmental variables rather than directly referencing them.
This code depends on 'secret_key':
key = ENV['secret_key']When 'secret_key' is not set, 'key' will be assigned nil, which is an object.
Later, a call to key.upcase will produce a NoMethodError, because Ruby doesn't know how to uppercase nil.
This is better:
key = ENV.fetch('secret_key')Now, when 'secret_key' is not set, KeyError will be raised. The code fails fast.
fetch takes a second argument as a default, which means you can even prevent KeyError entirely, if you choose.