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.

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

@@ -42,11 +42,11 @@ def owner_key_name
 
           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
 

activerecord/test/cases/associations/eager_test.rb

@@ -1218,6 +1218,7 @@ def test_preloading_empty_belongs_to
 
     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 @@ def test_preloading_empty_belongs_to_polymorphic
 
     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