Updated Rails flash messages using Bootstrap 4 from https://gist.github.com/roberto/3344628

_flash_messages.html.erb

<% flash.each do |type, message| %>
  <div class="alert alert-<%= bootstrap_class_for(type) %> fade in">
    <button class="close" data-dismiss="alert">×</button>
    <%= message %>
  </div>
<% end %>

application.html.erb

<%= render partial: "layouts/flash_messages", flash: flash %> 

application_controller.rb

class ApplicationController < ActionController::Base
  add_flash_types :success, :warning, :info, :error
end

application_helper.rb

module ApplicationHelper

  def bootstrap_class_for flash_type
    case flash_type
      when :success
        "alert-success"
      when :error
        "alert-error"
      when :alert
        "alert-block"
      when :info
        "alert-info"
      else
        flash_type.to_s
    end
  end

end