hash_for is no longer returning the :use_route key in the hash on 1.12, which causes some caching ("recall") in actionpack/lib/action_dispatch/routing/route_set.rb to try and prepend foreman_salt/ to the controller name, assuming that "hosts" is relative to the current foreman_salt/environments controller.

1.11 hash_for returns:

=> {:action=>"index", :controller=>"hosts", :use_route=>"hosts", :only_path=>true, :search=>"salt_environment = ..."}

develop:

=> {:controller=>"hosts", :action=>"index", :search=>"salt_environment = ..."}

Edit: not "caching" as I said above, but actually it's the current controller name/prefix that's being prepended to the :controller given in the hash. It's treated as a relative controller name unless use_route is also given.

Thanks for taking the time to test nightlies/1.12 and report this issue, I expect the fix will be in an early 1.12 RC. The one-line patch linked in the pull request can also easily be applied to /usr/share/foreman/config/initializers/routing_hash_for.rb.

Thanks for the quick work! Applying the patch on my end does indeed fix the problem.