Use DATABASE_URL instead of temp database.yml for multi-db tests

Replace the RAILS_DATABASE_YML / Dir.mktmpdir approach with DATABASE_URL:
- Read database config via Rails.application.config.database_configuration
  instead of manually parsing YAML+ERB
- Build a DATABASE_URL from each test config and pass it to subprocesses;
  Rails merges it on top of the test: entry — no temp files needed
- Remove non_test_configs, Dir.mktmpdir, and the require yaml/erb/tmpdir
- Remove RAILS_DATABASE_YML override from config/application.rb(.dist)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/database.yml.dist

@@ -48,3 +48,24 @@ production:
 #test:
 #  <<: *default
 #  database: fixinme_test
+
+# Multi-database testing
+# ----------------------
+# Any key starting with "test" is treated as a test database.
+# When more than one is present, EVERY test task (rails test, rails test:models,
+# rails test:system, …) automatically runs against all of them.
+#
+# The adapter gem must be available:
+#   bundle config --local with "mysql sqlite"   # mysql + sqlite
+#   bundle config --local with "mysql pg"       # mysql + postgresql
+#
+#test_sqlite:
+#  adapter: sqlite3
+#  database: db/fixinme_test.sqlite3
+#
+#test_pg:
+#  adapter: postgresql
+#  database: fixinme_test
+#  username: fixinme
+#  password: Some-password1%
+#  host: localhost

lib/tasks/test_databases.rake

@@ -0,0 +1,164 @@
+require 'uri'
+
+# Multi-database test runner
+# ==========================
+# When database.yml contains more than one `test*` configuration, every
+# standard test task (test, test:models, test:system, …) is automatically
+# rewritten to run the full suite against EACH configured database in turn.
+#
+# Convention — any top-level key that starts with "test" and holds a Hash:
+#
+#   test:           ← required primary database
+#     adapter: mysql2
+#     database: fixinme_test
+#     ...
+#
+#   test_sqlite:    ← optional additional databases
+#     adapter: sqlite3
+#     database: db/fixinme_test.sqlite3
+#
+#   test_pg:
+#     adapter: postgresql
+#     ...
+#
+# A single-database setup is unchanged: every task behaves exactly as before.
+#
+# Database configuration is read via Rails.application.config.database_configuration
+# so ERB interpolation and all Rails path overrides are respected automatically.
+# Each subprocess receives DATABASE_URL built from the selected config, which
+# Rails merges on top of the test: entry from database.yml — no temporary files needed.
+
+module MultiDbTests
+  ADAPTER_GEMS = {
+    'mysql2'      => 'mysql2',
+    'sqlite3'     => 'sqlite3',
+    'postgresql'  => 'pg',
+    'pg'          => 'pg',
+  }.freeze
+
+  ADAPTER_BUNDLE_GROUPS = {
+    'mysql2'      => 'mysql',
+    'sqlite3'     => 'sqlite',
+    'postgresql'  => 'postgresql',
+    'pg'          => 'postgresql',
+  }.freeze
+
+  # Rake task names generated by railties/lib/rails/test_unit/testing.rake
+  # that use run_from_rake — these are the ones we rewrite.
+  WRAPPED_TASKS = (
+    ['test'] +
+    Rails::TestUnit::Runner::TEST_FOLDERS.map { |f| "test:#{f}" } +
+    %w[test:all test:system test:generators test:units test:functionals]
+  ).freeze
+
+  class << self
+    # Returns {name => config_hash} for every key starting with "test".
+    def test_configs
+      @test_configs ||= Rails.application.config.database_configuration
+        .select { |k, v| k.to_s.start_with?('test') && v.is_a?(Hash) }
+    end
+
+    # Run rails +task_name+ for every configured test database.
+    # Called from the rewritten rake task actions.
+    def run(task_name)
+      cfgs    = test_configs
+      results = {}
+
+      cfgs.each do |db_name, config|
+        adapter = config['adapter'].to_s
+        puts "\n#{'─' * 64}"
+        puts "  #{task_name}  ·  #{db_name}  (#{adapter})"
+        puts '─' * 64
+
+        unless adapter_available?(adapter)
+          warn "  SKIPPED — '#{adapter}' gem not in bundle.\n" \
+               "  Run: bundle config --local with \"#{current_with} #{adapter_group(adapter)}\""
+          results[db_name] = :skipped
+          next
+        end
+
+        env = { 'DATABASE_URL' => config_to_url(config) }
+
+        if system(env, 'bundle exec rails db:test:prepare')
+          results[db_name] = system(env, 'rails', task_name) ? :pass : :fail
+        else
+          results[db_name] = :prepare_failed
+        end
+      end
+
+      print_summary(cfgs, results)
+
+      failed = results.count { |_, s| [:fail, :prepare_failed].include?(s) }
+      exit(false) if failed > 0
+    end
+
+    private
+
+    # Build a DATABASE_URL string from a database.yml config hash.
+    # Rails merges DATABASE_URL on top of the test: entry, so adapter and
+    # connection details from the URL take precedence over the YAML file.
+    def config_to_url(config)
+      adapter  = config['adapter'].to_s
+      database = config['database'].to_s
+
+      return "sqlite3:#{database}" if adapter == 'sqlite3'
+
+      user   = config['username']
+      pass   = config['password']
+      host   = config['host'] || 'localhost'
+      port   = config['port']
+      socket = config['socket']
+
+      userinfo = user ? "#{URI.encode_www_form_component(user)}:#{URI.encode_www_form_component(pass.to_s)}@" : ''
+      hostport = port ? "#{host}:#{port}" : host
+      url      = "#{adapter}://#{userinfo}#{hostport}/#{database}"
+      url     += "?socket=#{URI.encode_www_form_component(socket)}" if socket
+      url
+    end
+
+    def adapter_available?(adapter)
+      require ADAPTER_GEMS.fetch(adapter, adapter)
+      true
+    rescue LoadError
+      false
+    end
+
+    def adapter_group(adapter)
+      ADAPTER_BUNDLE_GROUPS.fetch(adapter, adapter)
+    end
+
+    def current_with
+      (Bundler.settings[:with] || '').split(':').join(' ')
+    end
+
+    def print_summary(cfgs, results)
+      return if results.size <= 1  # no summary for single-DB runs
+
+      puts "\n#{'═' * 64}"
+      puts '  SUMMARY'
+      puts '═' * 64
+      results.each do |db_name, status|
+        adapter = cfgs.dig(db_name, 'adapter') || '?'
+        icon    = status == :pass ? '✓' : (status == :skipped ? '–' : '✗')
+        label   = { pass: 'PASS', fail: 'FAIL',
+                    prepare_failed: 'PREPARE FAILED', skipped: 'SKIPPED' }[status]
+        puts "  #{icon}  #{db_name.ljust(26)} (#{adapter.ljust(12)})  #{label}"
+      end
+      puts '═' * 64
+    end
+  end
+end
+
+# Rewrite every standard test task to run against all configured databases.
+# This file loads after railties/testing.rake, so all tasks already exist.
+# Single-database setups are completely unaffected.
+if MultiDbTests.test_configs.size > 1
+  MultiDbTests::WRAPPED_TASKS.each do |task_name|
+    next unless Rake::Task.task_defined?(task_name)
+
+    Rake::Task[task_name].clear_actions
+    Rake::Task[task_name].enhance do
+      MultiDbTests.run(task_name)
+    end
+  end
+end