Fix duplicates in children count with multiple parents

arkindex/documents/api/elements.py

@@ -67,7 +67,7 @@ def _fetch_children_count(elements):
         ElementPath.objects
                    .filter(path__last__in=map(operator.attrgetter('id'), elements))
                    .values('path__last')
-                   .annotate(count=Count('path__last'))
+                   .annotate(count=Count('element_id', distinct=True))
                    .values_list('path__last', 'count')
     )
 

arkindex/documents/tests/test_elements_api.py

@@ -827,3 +827,31 @@ class TestElementsAPI(FixtureAPITestCase):
                 'Volume 1': 8,
             }
         )
+
+    def test_with_children_count_unique(self):
+        r"""
+        Ensure with_children_count only counts children elements once, not even when ElementPaths are duplicated.
+        Counts may have duplicates as soon as an element's parent has multiple parents:
+
+        A   B    Element | Path
+         \ /     C       | A
+          C      C       | B
+          |      D       | A C
+          D      D       | B C
+
+        Here, requesting the parents of D with children counts should return C with 1 child,
+        although there are two ElementPaths whose last ID the path is C's.
+        """
+        act = Element.objects.get(name='Act 1')
+        act.add_parent(Element.objects.get(name='Volume 2'))
+        surface = Element.objects.get(name='Surface A')
+        with self.assertNumQueries(7):
+            response = self.client.get(
+                reverse('api:elements-parents', kwargs={'pk': str(surface.id)}),
+                data={'with_children_count': True},
+            )
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        self.assertDictEqual(
+            {element['name']: element['children_count'] for element in response.json()['results']},
+            {'Act 1': 1},
+        )