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]

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -40,7 +40,11 @@ def associated_records_by_owner(preloader)
 
           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 @@ def associated_records_by_owner(preloader)
                 rhs_records
               end
             end
+          end.tap do
+            reset_association(middle_records, source_reflection.name, preload_scope)
           end
         end
 

activerecord/test/cases/associations/nested_through_associations_test.rb

@@ -579,6 +579,17 @@ def test_nested_has_many_through_should_not_be_autosaved
     assert !c.post_taggings.empty?
   end
 
+  def test_polymorphic_has_many_through_when_through_association_has_not_loaded
+    cake_designer = CakeDesigner.create!(chef: Chef.new)
+    drink_designer = DrinkDesigner.create!(chef: Chef.new)
+    department = Department.create!(chefs: [cake_designer.chef, drink_designer.chef])
+    Hotel.create!(departments: [department])
+    hotel = Hotel.includes(:cake_designers, :drink_designers).take
+
+    assert_equal [cake_designer], hotel.cake_designers
+    assert_equal [drink_designer], hotel.drink_designers
+  end
+
   def test_polymorphic_has_many_through_when_through_association_has_already_loaded
     cake_designer = CakeDesigner.create!(chef: Chef.new)
     drink_designer = DrinkDesigner.create!(chef: Chef.new)