title: Rails flash message not showing in redirect_to
body: <p>In one controller I have </p>
  
  <pre><code>flash[:error] = "Message"
  redirect_to :root
  </code></pre>
  
  <p>The :root is handled by another controller, the view has </p>
  
  <pre><code>&lt;% if flash[:error] %&gt;
    &lt;p&gt;&lt;%= flash[:error] %&gt;&lt;/p&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>But nothing is being shown.  I inserted &lt;%= debug controller.session %>, here's what I got</p>
  
  <pre><code>"flash"=&gt;#&lt;ActionDispatch::Flash::FlashHash:0x2e79208 @used=#&lt;Set: {}&gt;, @closed=false, @flashes={}, @now=nil&gt;}
  </code></pre>
  
  <p>What did I do wrong?</p>