This application is to show how Rails URL generation for engines mounted at the root path is broken in regards to subdirectories. It specifies the latest version of the Rails beta but this issue is also present in 5.1.4.

First install the needed gems with are default for `5.2.0 and then two test mountable engines. To install these gem dependencies and get the application ready to run, do:

bundleruby install
rake db:migrate

To run the application, do:

rails s

The application has a subdirectory specified in config/environments/development.rb of /v2. In addition, config.ru (the default rails s server) has been configured to serve from that subdirectory. I'll also note the following behavior is also observed on Phusion Passanger running via NGINX.

config/routes.rb has two engines mounted: one at the route and the other at a mounted namespace. These engines are identical except that one uses isolate_namespace and the other does not as I was testing to see if that had an effect (it did not). You can switch the routes around to verify this yourself.

Once the application is running, go to: http://localhost:3000/v2/test. That page will output what the following url helper generates for the following route:

<%= link_to test_engine.test_path, test_engine.test_path %>

That route will be missing the v2 subdirectory and thus will fail to work. Now let's try the other engine at http://localhost:3000/v2/mount/test2 and see what the following outputs:

<%= link_to test_engine2.test_path, test_engine2.test_path %>

This will generate a correct link that include the v2 subdirectory in it.

Routing helpers should take into consideration the config.relative_url_root setting when generating routes for engines mounted at the root of the application. You can play around with the mounting endpoints for this to see that there doesn't seem to be any way to get these to work right without using a non-root value in the mount point.