Eager loading won't mutate owner record

Since #31575, `BelongsToAssociation#target=` replaces owner record's
foreign key to fix an inverse association bug.

But the method is not only used for inverse association but also used
for eager loading/preloading, it caused some public behavior changes
(#32338, #32375).

To avoid any side-effect in loading associations, I reverted the
overriding `#target=`, then introduced `#inversed_from` to replace
foreign key in `set_inverse_instance`.

Closes #32375.

1 files changed, 8 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 5011a9bbde..b3f5de4c5f 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -44,6 +44,14 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_raise(frozen_error_class) { client.firm = Firm.new(name: "Firm") }
   end
 
+  def test_eager_loading_wont_mutate_owner_record
+    client = Client.eager_load(:firm_with_basic_id).first
+    assert_not_predicate client, :firm_id_came_from_user?
+
+    client = Client.preload(:firm_with_basic_id).first
+    assert_not_predicate client, :firm_id_came_from_user?
+  end
+
   def test_missing_attribute_error_is_raised_when_no_foreign_key_attribute
     assert_raises(ActiveModel::MissingAttributeError) { Client.select(:id).first.firm }
   end