Rails 6 database config gotcha

README.md

With the new multiple database support in Rails 6 connecting to more than one database from Rails becomes easier. There is a small gotcha to be aware of though. Take the following configuration:

development:
  primary:
    adapter: postgresql
    encoding: unicode
    pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
    host: <%= ENV["PG_HOST"] || "/tmp" %>
    username: <%= ENV["PRIMARY_USERNAME"] %>
    password: <%= ENV["PRIMARY_PASSWORD"] %>
    database: "primary"
  secondary:
    adapter: postgresql
    encoding: unicode
    pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
    host: <%= ENV["PG_HOST"] || "/tmp" %>
    username: <%= ENV["SECONDARY_USERNAME"] %>
    password: <%= ENV["SECONDARY_PASSWORD"] %>
    database: <%= ENV["SECONDARY_DATABASE"] %>

Now imagine you for some reason forget to set SECONDARY_DATABASE. In this case Rails will report that there is no configuration at all for the secondary database.