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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
  end

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

show.html.erb
  <% 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.

Namespace
Methods
C
N
Constants
KEY = 'action_dispatch.request.flash_hash'.freeze
 
Class Public methods
new(app)
# File actionpack/lib/action_dispatch/middleware/flash.rb, line 252
def initialize(app)
  @app = app
end
Instance Public methods
call(env)
# File actionpack/lib/action_dispatch/middleware/flash.rb, line 256
def call(env)
  @app.call(env)
ensure
  session    = Request::Session.find(env) || {}
  flash_hash = env[KEY]

  if flash_hash && (flash_hash.present? || session.key?('flash'))
    session["flash"] = flash_hash.to_session_value
    env[KEY] = flash_hash.dup
  end

  if (!session.respond_to?(:loaded?) || session.loaded?) && # (reset_session uses {}, which doesn't implement #loaded?)
    session.key?('flash') && session['flash'].nil?
    session.delete('flash')
  end
end