Even though your route maps to the new action of the users controller, your form needs to point to the URI pattern, in your case /signup
.
When in doubt, you can find the URI pattern and other information in your routes. Run rake routes
in your console and you'll see this:
signup GET|POST /signup(.:format) users#new
Rails generates helper methods to use in your views for each of the routes, based on the route prefix. In your case you can use the helper method signup_path
. To fix the problem, change your view code to this:
<%= form_for @user, url: signup_path do |f| %>
Check the Rails documentation for more information.