add tests for update too

tests/test_saving_related.py

@@ -54,3 +54,10 @@ async def test_model_relationship():
             assert ws.id == 1
             assert ws.topic == "Topic 1"
             assert ws.category.name == "Foo"
+
+            ws.topic = 'Topic 2'
+            await ws.update()
+
+            assert ws.id == 1
+            assert ws.topic == "Topic 2"
+            assert ws.category.name == "Foo"