Create connection.active_record notification and use that to ensure that lazy-

loaded model classes have their connections wrapped in transactions.

See #17776

In Rails 4 config.eager_load was changed to false in the test environment. This
means that model classes that connect to alternate databases with
establish_connection are not loaded at start up. If use_transactional_fixtures
is enabled, transactions are wrapped around the connections that have been
established only at the start of the test suite. So model classes loaded later
don't have transactions causing data created in the alternate database not to
be removed.

This change resolves that by creating a new connection.active_record
notification that gets fired whenever a connection is established. I then added
a subscriber after we set up transactions in the test environment to listen for
additional connections and wrap those in transactions as well.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -849,6 +849,21 @@ module ActiveRecord
 
         remove_connection(spec.name)
         owner_to_pool[spec.name] = ConnectionAdapters::ConnectionPool.new(spec)
+
+        message_bus = ActiveSupport::Notifications.instrumenter
+        payload = {
+          connection_id: object_id
+        }
+        if spec
+          payload[:class_name] = spec.name
+          payload[:config] = spec.config
+        end
+
+        message_bus.instrument('!connection.active_record', payload) do
+          owner_to_pool[spec.name] = ConnectionAdapters::ConnectionPool.new(spec)
+        end
+
+        owner_to_pool[spec.name]
       end
 
       # Returns true if there are any active connections among the connection

activerecord/lib/active_record/fixtures.rb

@@ -968,6 +968,7 @@ module ActiveRecord
       @fixture_cache = {}
       @fixture_connections = []
       @@already_loaded_fixtures ||= {}
+      @connection_subscriber = nil
 
       # Load fixtures once and begin transaction.
       if run_in_transaction?
@@ -977,10 +978,36 @@ module ActiveRecord
           @loaded_fixtures = load_fixtures(config)
           @@already_loaded_fixtures[self.class] = @loaded_fixtures
         end
+
+        # Begin transactions for connections already established
         @fixture_connections = enlist_fixture_connections
         @fixture_connections.each do |connection|
           connection.begin_transaction joinable: false
         end
+
+        # When connections are established in the future, begin a transaction too
+        @connection_subscriber = ActiveSupport::Notifications.subscribe('!connection.active_record') do |_, _, _, _, payload|
+          model_class = nil
+          begin
+            model_class = payload[:class_name].constantize if payload[:class_name]
+          rescue NameError
+            model_class = nil
+          end
+
+          if model_class
+            begin
+              connection = ActiveRecord::Base.connection_handler.retrieve_connection(model_class)
+            rescue ConnectionNotEstablished
+              connection = nil
+            end
+
+            if connection && !@fixture_connections.include?(connection)
+              connection.begin_transaction joinable: false
+              @fixture_connections << connection
+            end
+          end
+        end
+
       # Load fixtures for every test.
       else
         ActiveRecord::FixtureSet.reset_cache
@@ -995,6 +1022,7 @@ module ActiveRecord
     def teardown_fixtures
       # Rollback changes if a transaction is active.
       if run_in_transaction?
+        ActiveSupport::Notifications.unsubscribe(@connection_subscriber) if @connection_subscriber
         @fixture_connections.each do |connection|
           connection.rollback_transaction if connection.transaction_open?
         end

activerecord/test/cases/connection_pool_test.rb

@@ -341,6 +341,18 @@ module ActiveRecord
         end
       end
 
+      def test_connection_notification_is_called
+        payloads = []
+        subscription = ActiveSupport::Notifications.subscribe('!connection.active_record') do |name, started, finished, unique_id, payload|
+          payloads << payload
+        end
+        ActiveRecord::Base.establish_connection :arunit
+        assert_equal [:class_name, :config, :connection_id], payloads[0].keys.sort
+        assert_equal 'primary', payloads[0][:class_name]
+      ensure
+        ActiveSupport::Notifications.unsubscribe(subscription) if subscription
+      end
+
       def test_pool_sets_connection_schema_cache
         connection = pool.checkout
         schema_cache = SchemaCache.new connection

activerecord/test/cases/fixtures_test.rb

@@ -622,6 +622,46 @@ class TransactionalFixturesOnCustomConnectionTest < ActiveRecord::TestCase
   end
 end
 
+class TransactionalFixturesOnConnectionNotification < ActiveRecord::TestCase
+  self.use_transactional_tests = true
+  self.use_instantiated_fixtures = false
+
+  def test_transaction_created_on_connection_notification
+    connection = stub(:transaction_open? => false)
+    connection.expects(:begin_transaction).with(joinable: false)
+    fire_connection_notification(connection)
+  end
+
+  def test_notification_established_transactions_are_rolled_back
+    # Mocha is not thread-safe so define our own stub to test
+    connection = Class.new do
+      attr_accessor :rollback_transaction_called
+      def transaction_open?; true; end
+      def begin_transaction(*args); end
+      def rollback_transaction(*args)
+        @rollback_transaction_called = true
+      end
+    end.new
+    fire_connection_notification(connection)
+    teardown_fixtures
+    assert(connection.rollback_transaction_called, "Expected <mock connection>#rollback_transaction to be called but was not")
+  end
+
+  private
+
+    def fire_connection_notification(connection)
+      ActiveRecord::Base.connection_handler.stubs(:retrieve_connection).with(Book).returns(connection)
+      message_bus = ActiveSupport::Notifications.instrumenter
+      payload = {
+        class_name: 'Book',
+        config: nil,
+        connection_id: connection.object_id
+      }
+
+      message_bus.instrument('!connection.active_record', payload) {}
+    end
+end
+
 class InvalidTableNameFixturesTest < ActiveRecord::TestCase
   fixtures :funny_jokes
   # Set to false to blow away fixtures cache and ensure our fixtures are loaded