Don't unset foreign key when preloading missing record

When a belongs to association's target is set, its foreign key is now
updated to match the new target. This is the correct behaviour when a
new record is assigned, but not when the existing record is preloaded.

As long as we mark the association as loaded, we can skip setting the
target when the record is missing and avoid clobbering the foreign key.

2 files changed, 4 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 0f38d6bbda..d6f7359055 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -42,11 +42,11 @@ module ActiveRecord
 
           def associate_records_to_owner(owner, records)
             association = owner.association(reflection.name)
+            association.loaded!
             if reflection.collection?
-              association.loaded!
               association.target.concat(records)
             else
-              association.target = records.first
+              association.target = records.first unless records.empty?
             end
           end
 
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index f18c6177ac..6e0cf30092 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1218,6 +1218,7 @@ class EagerAssociationTest < ActiveRecord::TestCase
 
     client = assert_queries(2) { Client.preload(:firm).find(c.id) }
     assert_no_queries { assert_nil client.firm }
+    assert_equal c.client_of, client.client_of
   end
 
   def test_preloading_empty_belongs_to_polymorphic
@@ -1225,6 +1226,7 @@ class EagerAssociationTest < ActiveRecord::TestCase
 
     tagging = assert_queries(2) { Tagging.preload(:taggable).find(t.id) }
     assert_no_queries { assert_nil tagging.taggable }
+    assert_equal t.taggable_id, tagging.taggable_id
   end
 
   def test_preloading_through_empty_belongs_to