TIL: define custom flash types in Rails

We've been using a custom :warning flash type beyond Rails' default :notice and :alert, but passing it was cumbersome, had to put it in flash manually, the common way of just passing a key to redirect_to like redirect_to(root_path, warning: "Try again!") did not work.

Turns out you can define custom flash types and have Rails expose the convenience keys automagically.

class ApplicationController < ActionController::Base
  add_flash_types :warning