Add missing test case for default_isolation_level

Related to Pull Request: rails#54836

Author: chaadow

activerecord/test/cases/transaction_isolation_test.rb

@@ -88,6 +88,26 @@ class Tag2 < ActiveRecord::Base
       end
     end
 
+    test "default_isolation_level but transaction overrides isolation" do
+      assert_nil Tag.default_isolation_level
+
+      events = []
+      ActiveSupport::Notifications.subscribed(
+        -> (event) { events << event.payload[:sql] },
+        "sql.active_record",
+      ) do
+        Tag.with_default_isolation_level(:read_committed) do
+          assert_equal :read_committed, Tag.default_isolation_level
+
+          Tag.transaction(isolation: :repeatable_read) do
+            Tag.create!(name: "jon")
+          end
+        end
+      end
+
+      assert_begin_isolation_level_event(events, isolation: "REPEATABLE READ")
+    end
+
     # We are testing that a nonrepeatable read does not happen
     if ActiveRecord::Base.lease_connection.transaction_isolation_levels.include?(:repeatable_read)
       test "repeatable read" do
@@ -134,11 +154,11 @@ class Tag2 < ActiveRecord::Base
     end
 
     private
-      def assert_begin_isolation_level_event(events)
+      def assert_begin_isolation_level_event(events, isolation: "READ COMMITTED")
         if current_adapter?(:PostgreSQLAdapter)
-          assert_equal 1, events.select { _1.match(/BEGIN ISOLATION LEVEL READ COMMITTED/) }.size
+          assert_equal 1, events.select { _1.match(/BEGIN ISOLATION LEVEL #{isolation}/) }.size
         else
-          assert_equal 1, events.select { _1.match(/SET TRANSACTION ISOLATION LEVEL READ COMMITTED/) }.size
+          assert_equal 1, events.select { _1.match(/SET TRANSACTION ISOLATION LEVEL #{isolation}/) }.size
         end
       end
   end