You can configure the database in Rails via both the config/database.yml file and the DATABASE_URL env var. They are merged but settings from DATABASE_URL take precedence.
A good solution is to check in the config/database.yml with a basic configuration thats just enough to support postgres.app or whatever is the most common solution.
Developers that need other settings can set the DATABASE_URL env var to customize the database configuration. Gems like figaro and dotenv make this easier.
If you have ever wondered this how Heroku manages to use the correct DB no matter what you throw into database.yml and the DATABASE_URL ENV var is how you should be configuring your production DB.
Why should I use ENV vars and not more database_*.yaml files?
The twelve-factor app stores config in environment variables (often
  shortened to env vars or env). Env vars are easy to change between
  deploys without changing any code; unlike config files, there is
  little chance of them being checked into the code repo accidentally;
  and unlike custom config files, or other config mechanisms such as
  Java System Properties, they are a language- and OS-agnostic standard.
https://12factor.net/config