Bootstrap 4 alerts with rails 4.2

application_flashes.html.erb

<% if flash.any? %>
    <div class="row flashes">
      <div class="col-md-8 col-md-offset-2">
        <% flash.each do |msg_type, message| %>
            <%= flash_message(msg_type, message) %>
        <% end %>
      </div>
    </div>
<% end %>

application_helper.rb

module ApplicationHelper
  def bootstrap_alert_class_for(flash_type)
    {success: "alert-success",
     error: "alert-danger",
     alert: "alert-warning",
     notice: "alert-info"
    }[flash_type.to_sym] || flash_type.to_s
  end

  def flash_message(msg_type, message)
    concat(content_tag(:div, message, class: "alert #{bootstrap_alert_class_for(msg_type)} alert-dismissible fade in", role: "alert") do
      concat content_tag(:button, '×'.html_safe, class: "close", data: {dismiss: 'alert'}, 'aria-label': "Close")
      concat message
    end)
    nil
  end
end