Correct use of the flash in Rails

[Update 9 May 2012]

This seems to work for testing in Rails 3:

it "puts an error in the flash" do
  post :create
  flash[:error].should == "Sorry, that's wrong."

it "does not persist the flash" do
  post :create
  flash[:error].should be_nil

[Update 20 April 2010]

I recently had problems testing, and Google kept leading me back to this post. Unfortunately it doesn’t seem to work with the current version of Rails (I’m using 2.3.5 at the moment).

This post from Pluit Solutions gives an alternative approach which seems to work. I haven’t tried it with Rails 3 though.

I don’t know whether this has caught anyone else out, or whether we just didn’t read the documentation properly (it’s covered briefly on p153 of AWDwR), but I thought I’d mention it anyway.

Anyone who’s written a Rails app will know that the ‘flash’ is used to store error and status messages, usually on form submissions. Model validation failure messages automatically get copied into the flash, but you often want to do it manually too.

flash[:notice] = "User Details updated."
redirect_to edit_user_path(@user)

The gotcha comes when you want to display a message and render a page, as opposed to redirecting – for example when errors are preventing a form from being submitted. This is how not to do it:

flash[:error] = "Password doesn't match confirmation." # WRONG!
render :action => 'change_password'

The problem is that the flash is stored for the next request. Because we’re no longer doing a redirect, that means the message may appear wherever the user goes next, not just on the page that we just rendered. To avoid this, use, which is only used for the current request:[:error] = "Password doesn't match confirmation."
render :action => 'change_password'

The rule of thumb is to use flash if you’re redirecting, and if you’re rendering (either explicitly, or by dropping through to the default view for the action).

All very well, but whatever you put in is cleared out at the end of the request, so how do you test it? The answer (for RSpec, at least) lies in a comment on this RSpec feature request – basically just add the following to spec_helper.rb:

module ActionController
  module Flash
    class FlashHash 
      def initialize
        @hash = {}
        @now_hash = {}
      def [](key)
      def []=(key, obj)
        @hash[key] = obj
      def discard(k = nil)
      def now
      def update(hash)
      def sweep
        # do nothing

You can now do something like this:

describe "When a user tries to change his password with an invalid verification code" do

  it "should put an error message in the flash" do[:error].should == "Incorrect verification code or password."
  it "should not persist the flash" do
    flash[:error].should be_nil

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5 replies on “Correct use of the flash in Rails”


thanks – you’ve just helped me clear a pile of pending specs

and good to know that there’s at least one other rails/rspec fans at the labs



Thanks for this! Brilliant!

Have you thought of having FlashHash iteself extend Hash rather than reimplementing [] and []=? Was there any reason why you didn’t go that way?


This worked very well but when I tried to test a helper, I really couldn’t get it to work.

my test looked something like this:

it “should…”
flash[:warning] = “warning”

this didn’t set the flash in the helper when I used this solution. In the end I gave up and used the less pretty solution:

response.should have_tag(“div.warning”)

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