Rails Flash message is not shown

I have a line in a controller that will redirect to the home screen with a custom flash message, like so:

redirect_to :contr开发者_JAVA技巧oller => :home, :action => :index, :alert => "You are not yet assigned to an experiment." and return

The user is then redirected to the following page: http://localhost:3000/home?alert=You+are+not+yet+assigned+to+an+experiment

However, the flash message is not shown, although all other flash messages work in my application. The views/layouts/application.html.erb file contains the following:

<div class="flash">
    <%- if !notice.nil? -%>
    <p class="notice"><%= notice %></p>
    <%- end -%> 
    <%- if !alert.nil? -%>
    <p class="alert"><%= alert %></p>
    <%- end -%>     
</div>

What am I doing wrong here?

Flash is using session to store messages

flash[:alert] = "You are not yet assigned to an experiment."
redirect_to :controller => :home, :action => :index and return
<%= flash[:alert] %>

If you want to pass it through params you should use params[:param_name]

redirect_to :controller => :home, :action => :index, :alert => "You are not yet assigned to an experiment." and return
<%= params[:alert] %>

Flash is more preferable, it works implicitly across pages and is cleared after it's shown

In your view, you access a notice flash message by using

<%= flash[:notice] %>

not

<%= notice %>

The same applies to alert.

Also, there's an error in your code.

redirect_to :controller => :home, :action => :index, :alert => "You are not yet assigned to an experiment."

must be

redirect_to({ :controller => :home, :action => :index }, :alert => "You are not yet assigned to an experiment.")