Fix display set index with duplicate order (#4097)

This is not efficient, the original implementation iterated over all of
the display sets in the reader study and this keeps the same just to fix
the bug. It can probably be replaced with a window function.

Closes #4095

Author: jmsmkn

app/grandchallenge/reader_studies/models.py

@@ -987,13 +987,7 @@ def description(self) -> str:
 
     @property
     def standard_index(self) -> int:
-        return len(
-            [
-                x
-                for x in self.reader_study.display_sets.all()
-                if x.order < self.order
-            ]
-        )
+        return [*self.reader_study.display_sets.all()].index(self)
 
     @property
     def update_url(self):

app/grandchallenge/reader_studies/views.py

@@ -1022,7 +1022,10 @@ class DisplaySetViewSet(
         DisplaySet.objects.all()
         .select_related("reader_study__hanging_protocol")
         .prefetch_related(
-            "values__image", "values__interface", "reader_study__display_sets"
+            "values__image",
+            "values__interface",
+            "reader_study__display_sets",
+            "reader_study__optional_hanging_protocols",
         )
     )
     permission_classes = [DjangoObjectPermissions]

app/tests/reader_studies_tests/test_api.py

@@ -2093,6 +2093,31 @@ def test_display_set_index(client):
     ]
 
 
+@pytest.mark.django_db
+def test_display_set_index_with_duplicate_order(
+    client, django_assert_num_queries
+):
+    reader_study = ReaderStudyFactory()
+    user = UserFactory()
+    reader_study.add_reader(user)
+
+    ds1, ds2, *_ = DisplaySetFactory.create_batch(3, reader_study=reader_study)
+
+    ds2.order = ds1.order
+    ds2.save()
+
+    with django_assert_num_queries(34):
+        response = get_view_for_user(
+            viewname="api:reader-studies-display-set-list",
+            data={"reader_study": str(reader_study.pk)},
+            user=user,
+            client=client,
+            method=client.get,
+        )
+
+    assert [x["index"] for x in response.json()["results"]] == [0, 1, 2]
+
+
 @pytest.mark.django_db
 def test_total_edit_duration(client):
     rs = ReaderStudyFactory(allow_answer_modification=True)