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)
    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 %>

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.

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November 16, 2008 - (>= v2.1.0)
11 thanks

Application Helper for Fading Flash Messages

A simple helper method for showing the flash message. Includes optional fade in seconds (view needs javascript_include_tag defaults if you desire fade effect):

def show_flash_message(options={})
  html = content_tag(:div, flash.collect{ |key,msg| content_tag(:div, msg, :class => key) }, :id => 'flash-message')
  if options.key?(:fade)
    html << content_tag(:script, "setTimeout(\"new Effect.Fade('flash-message');\",#{options[:fade]*1000})", :type => 'text/javascript')
  end
  html
end

simply call in your views then using:

<%= show_flash_message(:fade => 4) %>
June 11, 2009
4 thanks

Keeping the flash object on multiple redirects

If your controllers are redirecting more than once, the flash contents will be lost. To avoid it, execute flash.keep before each redirection.

Check ActionController::Flash::FlashHash for more handy methods (discard, now, …)

March 20, 2009
2 thanks

Broadened Flash helper

Building on the below excellent example, you can create something with default options for how long it’s displayed and how long the fade is, and highlight:

def show_flash(options={})
  options = {:fade => 3, :display => 3, :highlight => true}.merge(options)
  html = content_tag(:div, flash.collect{ |key,msg| content_tag(:div, msg, :class => key, :attributes => "style = display: none;") }, :id => 'flash-message')
  html << content_tag(:script, "new Effect.Highlight('flash-message');") if options[:highlight]
  html << content_tag(:script, "$('flash-message').appear();")
  html << content_tag(:script, "setTimeout(\"$('flash-message').fade({duration: #{options[:fade]}});\", #{options[:display]*1000});")
end
August 5, 2014
0 thanks

Handling flash for ajax calls

This is helper method which can be easily used to handle flash for ajax calls

##call it in your js.erb
def flash_display
response = ""
flash.each do |name, msg|
	msg=msg+"<button type='button' class='close' title='hide' data- dismiss='alert'><i class='fa-times-circle-o fa pull-right'></i></button>".html_safe
  response = response + 
  content_tag(:div, msg, :id => "flash_# {name}",:class=>"alert alert-danger") do 
  	"#{msg}".html_safe  
  end 	
end
flash.discard
response
end

so in your controller,

def get_data_and_update_div
 @user_details=User.get_details
 if @user_details.nil?
   flash.now[:error]="Add your details and then submit"
   format.js { render   'shared/error_messages'}
 end

in error_messages.js.erb

$('#flash_messages').html("<%= escape_javascript raw(flash_display) %>");

in view file

<div id="flash_messages">
  <% if flash[:error] %>
   <%= content_tag :div, flash['value'], :id =>"flash_error" %>
 <% end %>
</div>