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>

lib/tasks/test_databases.rake

@@ -1,6 +1,4 @@
-require 'yaml'
-require 'erb'
-require 'tmpdir'
+require 'uri'
 
 # Multi-database test runner
 # ==========================
@@ -25,9 +23,10 @@ require 'tmpdir'
 #
 # 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.
+# 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 = {
@@ -55,19 +54,8 @@ module MultiDbTests
   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
+      @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.
@@ -89,16 +77,12 @@ module MultiDbTests
           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 }
+        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
+        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
 
@@ -110,6 +94,28 @@ module MultiDbTests
 
     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