Creating A Test DB in Dockerized Rails

I ran into all sorts of trouble when trying to create a simple test database for a Dockerized Rails app.

Maybe Docker Compose was not passing the environmental variable RAILS_ENV=test?

$ docker compose exec -e RAILS_ENV=test web bash 
root@b1c40fe16325:/rails# echo $RAILS_ENV 
test

Maybe Rails wasn’t picking up RAILS_ENV=test?

Loading test environment (Rails 7.1.5) 
[1] pry(main)> Rails.env 
=> "test" 

After many frustrating, useless iterations with ChatGPT, I took the time to RTFM:

$ docker compose exec -e RAILS_ENV=test web rails -T | grep "db:create"

bin/rails db:create # Create the database from DATABASE_URL or config/database.yml for the current RAILS_ENV (use db:create:all to create all databases in the config). Without RAILS_ENV or when RAILS_ENV is development, it defaults to creating the development and test databases, except when DATABASE_URL is present

The key is here:

it defaults to creating the development and test databases, except when DATABASE_URL is present

And what was in docker-compose.yml? You guessed it:

- DATABASE_URL=postgres://...

Which means:

1. Rails_ENV=test should not be necessary for creating a test DB, because it is the default behaviour of rails db:create.
2. If DATABASE_URL is set in docker-compose.yml, Rails will ignore RAILS_ENV anyway.

Therefore, to create a test DB, I simply removed the DATABASE_URL from docker-compose.yml.