Added a testcase for rails#5563

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -704,4 +704,15 @@ def test_polymorphic_with_custom_primary_key
 
     assert_equal toy, sponsor.reload.sponsorable
   end
+
+  def test_reflect_the_most_recent_change
+    author1, author2 = Author.limit(2)
+    post = Post.new(:title => "foo", :body=> "bar")
+
+    post.author    = author1
+    post.author_id = author2.id
+
+    assert post.save
+    assert_equal post.author_id, author2.id
+  end
 end