Fix derived union propagation issue in case of one association

gaphor/UML/properties.py

@@ -743,21 +743,18 @@ def _union(self, obj, exclude=None):
         """
         Returns a union of all values as a set.
         """
-        if self.single:
-            return next(iter(self.subsets)).__get__(obj)
-        else:
-            u: Set[T] = set()
-            for s in self.subsets:
-                if s is exclude:
-                    continue
-                tmp = s.__get__(obj)
-                if tmp:
-                    try:
-                        u.update(tmp)
-                    except TypeError:
-                        # [0..1] property
-                        u.add(tmp)
-            return collectionlist(u)
+        u: Set[T] = set()
+        for s in self.subsets:
+            if s is exclude:
+                continue
+            tmp = s.__get__(obj)
+            if tmp:
+                try:
+                    u.update(tmp)
+                except TypeError:
+                    # [0..1] property
+                    u.add(tmp)
+        return collectionlist(u)
 
     def propagate(self, event):
         """

gaphor/UML/tests/test_properties.py

@@ -453,24 +453,44 @@ class A(Element):
     assert d in a.u
 
 
-@pytest.mark.skip
-def test_derivedunion_notify():
+def test_derivedunion_notify_for_single_derived_property():
     class A(Element):
         pass
 
     class E(Element):
         notified = False
 
-        def notify(self, name, pspec):
-            if name == "u":
+        def handle(self, event):
+            if event.property is E.u:
                 self.notified = True
 
     E.a = association("a", A)
     E.u = derivedunion(E, "u", A, 0, "*", E.a)
 
     e = E()
-    assert e.notified is False
     e.a = A()
+
+    assert e.notified is True
+
+
+def test_derivedunion_notify_for_multiple_derived_properties():
+    class A(Element):
+        pass
+
+    class E(Element):
+        notified = False
+
+        def handle(self, event):
+            if event.property is E.u:
+                self.notified = True
+
+    E.a = association("a", A)
+    E.aa = association("aa", A)
+    E.u = derivedunion(E, "u", A, 0, "*", E.a, E.aa)
+
+    e = E()
+    e.a = A()
+
     assert e.notified is True