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.

2 files changed, 43 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index 9b74c3a10f..1d9579f1a8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/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
diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index 51bf12d0bf..6d142285cd 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/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