How to point Rails in `development` to a differently named DB config in `database.yml`?

Question

The goal is to configure all 3 default environments ('development', 'test', 'production') from the ENV hash. How can I tell Rails to establish the following mapping, where main is defined in config/database.yml below?

'development' -> main
'test' -> main
'production' -> main

DB configs are consolidated to main in config/database.yml like so:

defaults: &defaults
  adapter: postgresql
  encoding: utf8
  port: 5432
  timeout: 10000

main:
  <<: *defaults
  url: <%= ENV['MAIN_DATABASE_URL'] %>
  pool: <%= ENV['MAIN_DATABASE_POOL'] %>

In :development & :test, gem 'dotenv-rails', groups: [:development, :test] loads the runtime env from .env files. In :production the ENV is set via other means.

Alas, when I do rails s, this errors:

.../activerecord-5.2.0/lib/active_record/connection_adapters/connection_specification.rb:260:in `resolve_symbol_connection':
'development' database is not configured. Available: ["defaults", "main"] (ActiveRecord::AdapterNotSpecified)

And the docs on DB configuration don't have an answer.

Answer 1

If you add an anchor to main, e.g. &main:

main: &main
  <<: *defaults
  url: <%= ENV['MAIN_DATABASE_URL'] %>
  pool: <%= ENV['MAIN_DATABASE_POOL'] %>

You can use it to define your environments:

development: *main
test: *main
production: *main

or, just like you did with defaults:

development:
  <<: *main

test:
  <<: *main

production:
  <<: *main

The latter allows you to add additional key-value pairs or to override existing ones.