The flash provides a way to pass temporary objects between actions. Anything you place in the flash will be exposed to the very next action and then cleared out. This is a great way of doing notices and alerts, such as a create action that sets flash[:notice] = "Successfully created" before redirecting to a display action that can then expose the flash to its template. Actually, that exposure is automatically done. Example:

  class PostsController < ActionController::Base
    def create
      # save post
      flash[:notice] = "Successfully created post"
      redirect_to posts_path(@post)

    def show
      # doesn't need to assign the flash notice to the template, that's done automatically

    <% if flash[:notice] %>
      <div class="notice"><%= flash[:notice] %></div>
    <% end %>

This example just places a string in the flash, but you can put any object in there. And of course, you can put as many as you like at a time too. Just remember: They‘ll be gone by the time the next action has been performed.

See docs on the FlashHash class for more details about the flash.

Included Modules
Classes and Modules
Class ActionController::Flash::FlashHash
Public Class methods
    # File actionpack/lib/action_controller/flash.rb, line 29
29:     def self.included(base)
30:       base.class_eval do
31:         include InstanceMethods
33:         alias_method_chain :perform_action, :flash
34:         alias_method_chain :reset_session,  :flash
35:         alias_method_chain :redirect_to,    :flash
37:         helper_method :alert
38:         helper_method :notice
39:       end
40:     end