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

@@ -20,6 +20,10 @@ Bundler.require(*Rails.groups)
 
 module FixinMe
   class Application < Rails::Application
+    # Allow RAILS_DATABASE_YML to override the database config file path.
+    # 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.load_defaults 7.0
 

config/database.yml.dist

@@ -48,3 +48,26 @@ production:
 #test:
 #  <<: *default
 #  database: fixinme_test
+
+# 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
+#
+# 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
+#  username: fixinme
+#  password: Some-password1%
+#  host: localhost

lib/tasks/test_databases.rake

@@ -0,0 +1,118 @@
+require 'yaml'
+require 'erb'
+require 'tmpdir'
+
+namespace :test do
+  desc <<~DESC
+    Run the full test suite against every test database configured in database.yml.
+
+    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',
+  'postgresql' => 'pg',
+  'pg'         => 'pg',
+}.freeze
+
+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
+
+def adapter_group(adapter)
+  ADAPTER_GROUPS.fetch(adapter, adapter)
+end