Fix preloading polymorphic multi-level through association

This is partially fixed by e617fb57 when through association has already
loaded. Otherwise, second level through association should respect
`preload_scope`.

Fixes #30242.
Closes #30076.

[Ryuta Kamizono & CicholGricenchos]

1 files changed, 7 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 5bd49b041a..b16fca7dc9 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -40,7 +40,11 @@ module ActiveRecord
 
           middle_records = through_records.flat_map(&:last)
 
-          reflection_scope = reflection_scope() if reflection.scope
+          if preload_scope
+            reflection_scope = reflection_scope().merge(preload_scope)
+          elsif reflection.scope
+            reflection_scope = reflection_scope()
+          end
 
           preloaders = preloader.preload(middle_records,
                                          source_reflection.name,
@@ -70,6 +74,8 @@ module ActiveRecord
                 rhs_records
               end
             end
+          end.tap do
+            reset_association(middle_records, source_reflection.name, preload_scope)
           end
         end