Fix #5563. Should reflect the most recent change to either of association / id.

activerecord/lib/active_record/associations/association.rb

@@ -132,7 +132,8 @@ def target_scope
       # ActiveRecord::RecordNotFound is rescued within the method, and it is
       # not reraised. The proxy is \reset and +nil+ is the return value.
       def load_target
-        @target ||= find_target if find_target?
+        @target = find_target if (@stale_state && stale_target?) || find_target?
+
         loaded! unless loaded?
         target
       rescue ActiveRecord::RecordNotFound

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -705,4 +705,15 @@ 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