Fix preloading polymorphic association when through association has already loaded

If through association has already loaded, `source_type` is ignored to
loaded through records. The loaded records should be filtered by
`source_type` in that case.

Fixes #30904.

1 files changed, 11 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/nested_through_associations_test.rb b/activerecord/test/cases/associations/nested_through_associations_test.rb
index 4cf5a9ffc4..11ee45917f 100644
--- a/activerecord/test/cases/associations/nested_through_associations_test.rb
+++ b/activerecord/test/cases/associations/nested_through_associations_test.rb
@@ -579,6 +579,17 @@ class NestedThroughAssociationsTest < ActiveRecord::TestCase
     assert !c.post_taggings.empty?
   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)
+    department = Department.create!(chefs: [cake_designer.chef, drink_designer.chef])
+    Hotel.create!(departments: [department])
+    hotel = Hotel.includes(:chefs, :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_joined_different_table_twice
     cake_designer = CakeDesigner.create!(chef: Chef.new)
     drink_designer = DrinkDesigner.create!(chef: Chef.new)