Fix undo of attributes and enumerations

By setting the default value on unlink.

Fixes #643.

gaphor/core/modeling/properties.py

@@ -204,9 +204,7 @@ class attribute(umlproperty, Generic[T]):
             raise TypeError(error_msg)
 
     def unlink(self, obj):
-        old = self._get(obj)
-        log.debug("Unlink attribute %s.%s", obj, self.name)
-        self.handle(AttributeUpdated(obj, self, old, old))
+        self._set(obj, self.default)
 
     def __str__(self):
         return f"<attribute {self.name}: {self.type} = {self.default}>"
@@ -282,8 +280,7 @@ class enumeration(umlproperty):
         setattr(obj, self._name, value)
 
     def unlink(self, obj):
-        old = self._get(obj)
-        self.handle(AttributeUpdated(obj, self, old, old))
+        self._set(obj, self.default)
 
     def _set(self, obj, value):
         if value not in self.values:

tests/test_undo.py

@@ -92,19 +92,24 @@ def test_class_association_undo_redo(event_manager, element_factory, undo_manage
         undo_manager.redo_transaction()
 
 
-def test_diagram_item_can_undo_(event_manager, element_factory, undo_manager, caplog):
+def test_diagram_item_can_undo_and_redo(
+    event_manager, element_factory, undo_manager, caplog
+):
     caplog.set_level(logging.INFO)
     with Transaction(event_manager):
         diagram = element_factory.create(Diagram)
 
     with Transaction(event_manager):
         cls = diagram.create(ClassItem, subject=element_factory.create(UML.Class))
+        cls.subject.name = "name"
         cls.matrix.translate(10, 10)
 
     undo_manager.undo_transaction()
     undo_manager.redo_transaction()
-
-    assert diagram.ownedPresentation[0].matrix.tuple() == (1, 0, 0, 1, 10, 10)
+    new_cls = diagram.ownedPresentation[0]
+    assert new_cls.matrix.tuple() == (1, 0, 0, 1, 10, 10)
+    assert new_cls.subject, element_factory.select()
+    assert new_cls.subject.name == "name"
     assert not caplog.records