How to properly diagnose a 500 error (Rails, Passenger, Nginx, Postgres)

Okay, I figured this out. The app ran fine in development mode, so I knew something production-specific was screwing it up. I went into config/environments/production.rb and changes these settings:

# Full error reports are disabled and caching is turned on
config.consider_all_requests_local       = false # changed from true
config.action_controller.perform_caching = true # changed from false

And then after restarting passenger, rails showed me the error w/ stacktrace. Turns out I was forgetting to precompile the asset pipeline!

Things to check

1) Are you sure you are running in production environment? Check to see if any entries are in the development.log file

2) Set up your app to email you when a 500 error occurs with a full stack trace. I use the Exception Notifier gem but there are plenty of other solutions for this.

3) When checking your app in the console are you sure you are starting the console in production mode? It is possible that the app is not starting up at all and you just forgot to set the production param thereby thinking that the app runs fine when it doesn't.

4) Are you getting an nginx 500 error or the Rails 500 error? If nginx then it is likely that your app is not starting at all and highly unlikely that you will get any rails error in your log file. The assets are static files and navigating to them proves nothing other than that they exist.

5) Are you sure you are checking the right folder on the server? Sounds really stupid but capistrano could be deploying the app to a folder that is different to the folder that nginx is looking for for your app so double check both the folder capistrano is deploying to and the folder that nginx is looking for are the same.

Just a suggestion, I would use puma rather than passenger. It's awesome with nginx.

My problem is passenger's log file (error.log) has nothing. Then it's a rotation log issue. Run

 passenger-config reopen-logs

solved my problem. More.