Set flash message by convention with i18n

application.rb

class ApplicationController < ActionController::Base
  # Sets a flash message based on locale.
  def set_flash(key, options = {})
    case key
    when :notice, true
      flash[:notice] = flash_message(:notice, options)
    when :error, false
      flash[:error] = flash_message(:error, options)
    else
      raise ArgumentError.new "Unrecognized flash key: #{key}"
    end
  end

  def flash_message(key, options = {})
    options[:scope] ||= [:controllers, params[:controller], params[:action]]
    options[:default] ||= translate(key, scope: [:controllers])
    translate key, options
  end
end

en.yml

en:
  controllers:
    notice: "Default notice"
    error: "Default error"
    example:
      somthing:
        notice: "Something was done to %{foo} successfully"
        error: "There was a problem with %{foo}"

example_controller.rb

class ExampleController < ApplicationController
  def something
    # Do something
    success = something(params[:foo])
    # Set flash
    set_flash success, params
    # Redirect
    redirect_to example_index_url
  end
end

example_controller_test.rb

require 'test_helper'

class ExampleControllerTest < ActionController::TestCase
  test "something" do
    put :something, foo: "bar"
    assert_redirected_to example_index_path
    assert_flash :notice, foo: "bar"
  end
end

test_helper.rb

class ActiveSupport::TestCase
  # Asserts the correct flash message was set.
  def assert_flash(key, options = {})
    assert_equal flash[key], @controller.send(:flash_message, :notice, options)
  end
end