Add multi-database test runner (test:all_databases)

Adds `bundle exec rails test:all_databases` which runs the full test
suite against every test database configured in database.yml in a single
command.

Convention: any top-level key starting with "test" that contains a Hash
is a test database. `test:` is the required primary; `test_<name>:` blocks
are optional additional adapters (e.g. test_sqlite, test_pg).

For each configured database the task:
  1. Checks the required adapter gem is available (skips with warning if not)
  2. Runs `rails db:test:prepare` to create and migrate the database
  3. Runs `rails test` and records pass/fail
  4. Prints a summary and exits non-zero if any database failed

Mechanism: a RAILS_DATABASE_YML env var points each subprocess to a
temporary database.yml that contains only the current test config.
config/application.rb(.dist) reads this var and overrides Rails'
database config path before initialisation, so no monkey-patching of
the test runner is required.

config/database.yml.dist is updated with documented examples for SQLite
and PostgreSQL additional test databases.

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

config/application.rb.dist

@@ -21,7 +21,7 @@ Bundler.require(*Rails.groups)
 module FixinMe
   class Application < Rails::Application
     # Allow RAILS_DATABASE_YML to override the database config file path.
-    # Used by the multi-database test runner (lib/tasks/test_databases.rake).
+    # Used by `rails test:all_databases` to test against multiple DB adapters.
     config.paths['config/database'] = [ENV['RAILS_DATABASE_YML']] if ENV['RAILS_DATABASE_YML']
 
     # Initialize configuration defaults for originally generated Rails version.

config/database.yml.dist

@@ -49,20 +49,22 @@ production:
 #  <<: *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.
+# Multi-database testing — `bundle exec rails test:all_databases`
+# ---------------------------------------------------------------
+# Add any number of `test_<name>:` blocks to run the full test suite
+# against additional database adapters in a single command.
+# Each adapter's gem must be available in the bundle:
+#   bundle config --local with "mysql:sqlite"   # mysql + sqlite
+#   bundle config --local with "mysql:pg"       # mysql + postgresql
 #
-# The adapter gem must be available:
-#   bundle config --local with "mysql sqlite"   # mysql + sqlite
-#   bundle config --local with "mysql pg"       # mysql + postgresql
+# Example — run tests against MySQL and SQLite:
 #
 #test_sqlite:
 #  adapter: sqlite3
 #  database: db/fixinme_test.sqlite3
 #
+# Example — run tests against MySQL and PostgreSQL:
+#
 #test_pg:
 #  adapter: postgresql
 #  database: fixinme_test

lib/tasks/test_databases.rake

@@ -2,34 +2,94 @@ 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.
+namespace :test do
+  desc <<~DESC
+    Run the full test suite against every test database configured in database.yml.
 
-module MultiDbTests
+    Any top-level key that starts with "test" and contains a Hash is treated as a
+    test database configuration:
+
+      test:           # always required — the primary test database
+        adapter: mysql2
+        database: fixinme_test
+        ...
+
+      test_sqlite:    # optional additional databases
+        adapter: sqlite3
+        database: db/fixinme_test.sqlite3
+
+      test_pg:
+        adapter: postgresql
+        database: fixinme_test
+        ...
+
+    For each database the task will:
+      1. Check that the required adapter gem is available (skip with warning if not).
+      2. Run `rails db:test:prepare` to create/migrate the database.
+      3. Run `rails test` and record pass/fail.
+
+    A summary is printed at the end. The task exits non-zero if any database fails.
+  DESC
+  task :all_databases do
+    db_file    = Rails.root.join('config', 'database.yml')
+    all        = YAML.safe_load(ERB.new(db_file.read).result, aliases: true) || {}
+    test_cfgs  = all.select { |k, v| k.to_s.start_with?('test') && v.is_a?(Hash) }
+    non_test   = all.reject { |k, _| k.to_s.start_with?('test') }
+
+    abort "No test database configurations found in #{db_file}." if test_cfgs.empty?
+
+    results = {}
+
+    test_cfgs.each do |name, config|
+      adapter = config['adapter'].to_s
+
+      puts "\n#{'─' * 64}"
+      puts "  #{name}  (adapter: #{adapter})"
+      puts '─' * 64
+
+      unless adapter_available?(adapter)
+        puts "  SKIPPED — gem for '#{adapter}' adapter not available in bundle."
+        puts "  Add it to Gemfile (e.g. `bundle config --local with #{adapter_group(adapter)}`)"
+        results[name] = :skipped
+        next
+      end
+
+      Dir.mktmpdir('rails_test_db_') do |tmpdir|
+        tmp_yml = File.join(tmpdir, 'database.yml')
+        # Write a standalone database.yml with just this config as `test:`
+        # (non-test configs are preserved so boot doesn't fail on `production:` lookup)
+        File.write(tmp_yml, non_test.merge('test' => config).to_yaml)
+
+        env = { 'RAILS_DATABASE_YML' => tmp_yml }
+
+        if system(env, 'bundle exec rails db:test:prepare')
+          results[name] = system(env, 'bundle exec rails test') ? :pass : :fail
+        else
+          results[name] = :prepare_failed
+        end
+      end
+    end
+
+    puts "\n#{'═' * 64}"
+    puts "  SUMMARY"
+    puts '═' * 64
+    results.each do |name, status|
+      adapter = test_cfgs[name]['adapter']
+      icon    = { pass: '✓', fail: '✗', prepare_failed: '✗', skipped: '–' }[status]
+      label   = { pass: 'PASS', fail: 'FAIL',
+                  prepare_failed: 'PREPARE FAILED', skipped: 'SKIPPED' }[status]
+      puts "  #{icon}  #{name.ljust(26)} (#{adapter.ljust(12)})  #{label}"
+    end
+    puts '═' * 64
+
+    failed = results.count { |_, s| s == :fail || s == :prepare_failed }
+    abort "\n  #{failed} database(s) failed." if failed > 0
+  end
+end
+
+private
+
+# Returns true if the gem required for this adapter is loadable.
 ADAPTER_GEMS = {
   'mysql2'     => 'mysql2',
   'sqlite3'    => 'sqlite3',
@@ -37,122 +97,22 @@ module MultiDbTests
   'pg'         => 'pg',
 }.freeze
 
-  ADAPTER_BUNDLE_GROUPS = {
+def adapter_available?(adapter)
+  gem_name = ADAPTER_GEMS.fetch(adapter, adapter)
+  require gem_name
+  true
+rescue LoadError
+  false
+end
+
+# Returns the Bundler group name that installs the adapter gem.
+ADAPTER_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
+  ADAPTER_GROUPS.fetch(adapter, adapter)
 end