Extend all test tasks to run against every configured test database

When database.yml defines more than one top-level key starting with "test",
every standard test task (rails test, rails test:models, rails test:system,
rails test:controllers, …) is automatically rewritten at load time to run
the full suite against each configured database in turn.

Single-database setups are completely unaffected — the wrapping code
activates only when test_configs.size > 1, so existing behaviour is
preserved by default.

Convention: test: is the required primary; test_<name>: adds an adapter:
  test_sqlite:
    adapter: sqlite3
    database: db/fixinme_test.sqlite3
  test_pg:
    adapter: postgresql
    ...

Mechanism:
- lib/tasks/test_databases.rake loads after railties/testing.rake (which
  defines all test tasks). For each task in the wrapped set it calls
  Rake::Task[name].clear_actions and re-enhances with a block that loops
  over DB configs, writing a temporary database.yml per database and
  running "rails <task_name>" as a subprocess with RAILS_DATABASE_YML set.
- config/application.rb(.dist): reads RAILS_DATABASE_YML and overrides
  Rails' database config path before initialisation — no monkey-patching
  of the test runner required.
- Adapter gem availability is checked before each run; missing adapters are
  skipped with a hint on which bundle group to enable.
- A formatted summary (✓/✗ per database) is printed; exits non-zero on
  any failure.

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

lib/tasks/test_databases.rake

@@ -0,0 +1,158 @@
+require 'yaml'
+require 'erb'
+require 'tmpdir'
+
+# 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.
+#
+# The mechanism uses RAILS_DATABASE_YML — an env var read by
+# config/application.rb(.dist) to override Rails' database config path before
+# initialisation, giving each subprocess a clean, isolated database config.
+
+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 ||= begin
+        db_file = Rails.root.join('config', 'database.yml')
+        all     = YAML.safe_load(ERB.new(db_file.read).result, aliases: true) || {}
+        all.select { |k, v| k.to_s.start_with?('test') && v.is_a?(Hash) }
+      end
+    end
+
+    def non_test_configs
+      @non_test_configs ||= begin
+        db_file = Rails.root.join('config', 'database.yml')
+        all     = YAML.safe_load(ERB.new(db_file.read).result, aliases: true) || {}
+        all.reject { |k, _| k.to_s.start_with?('test') }
+      end
+    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
+
+        Dir.mktmpdir('rails_test_') do |tmpdir|
+          tmp_yml = File.join(tmpdir, 'database.yml')
+          File.write(tmp_yml, non_test_configs.merge('test' => config).to_yaml)
+          env = { 'RAILS_DATABASE_YML' => tmp_yml }
+
+          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
+      end
+
+      print_summary(cfgs, results)
+
+      failed = results.count { |_, s| [:fail, :prepare_failed].include?(s) }
+      exit(false) if failed > 0
+    end
+
+    private
+
+    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