subdir in nginx for rails api-app - ruby-on-rails

Question

I am trying to configure the nginx to have a separate subdir for rails-5 api backend. (to separate frontend and backend)
Original, I am calling backend api under GET "/bills". Now I'd like to be it: GET '/api/bills'. so all requests under 'api' should be redirected to rails app.
But I can't make it working. The redirection works, but I see on rails side in logs: ActionController::RoutingError (No route matches [GET] "/api/bills"). Of course this route doesn't exists. Rails only knows about the "/bills" route. Could I configure nginx so, that the redirection would be transparent to Rails, and it would see the request like [GET]"/bills" ?
here is my current config:
upstream app {
# Path to Unicorn SOCK file, as defined previously
server unix:/var/sockets/unicorn.myapp.sock fail_timeout=0;
}
server {
#redirect to https
listen 0.0.0.0:80;
listen [::]:80 ipv6only=on default_server;
server_name localhost; ## Replace this with something like gitlab.example.com
server_tokens off; ## Don't show the nginx version number, a security best practice
return 301 https://$server_name$request_uri;
access_log /var/log/nginx/app_access.log;
error_log /var/log/nginx/app_error.log;
}
server {
listen 0.0.0.0:443 ssl;
listen [::]:443 ipv6only=on ssl default_server;
server_name localhost; ## Replace this with something like gitlab.example.com
ssl on;
ssl_certificate /etc/ssl/nginx/host.crt;
ssl_certificate_key /etc/ssl/nginx/host.key;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_session_cache shared:SSL:10m;
ssl_session_timeout 5m;
location ^~ /api {
#try_files $uri/index.html $uri $uri/;
try_files $uri/index.html $uri #app;
root /app/backend/public;
}
location #app {
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
proxy_pass http://app;
error_page 500 502 503 504 /500.html;
}
location / {
# Application Frontend root, as defined previously
try_files $uri $uri/ =404;
root /app/frontend/;
}
client_max_body_size 4G;
keepalive_timeout 10;
}

Answer 1

Inside your location #app block try adding this:
rewrite ^/api(.*)$ $1 break;
That should just strip off the /api prefix before sending the remainder of the URI upstream.
See this document for details.