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The flash provides a way to pass temporary primitive-types (String, Array, Hash) 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] = "Post successfully created" before redirecting to a display action that can then expose the flash to its template. Actually, that exposure is automatically done.

class PostsController < ActionController::Base
  def create
    # save post
    flash[:notice] = "Post successfully created"
    redirect_to @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 %>

Since the notice and alert keys are a common idiom, convenience accessors are available:

flash.alert = "You must be logged in"
flash.notice = "Post successfully created"

This example places a string in the flash. And of course, you can put as many as you like at a time too. If you want to pass non-primitive types, you will have to handle that in your application. Example: To show messages with links, you will have to use sanitize helper.

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.

KEY = 'action_dispatch.request.flash_hash'.freeze
Class Public methods
# File actionpack/lib/action_dispatch/middleware/flash.rb, line 286
def self.new(app) app; end