Fix permission filtering for viewing own grades

aleksis/apps/maka/schema/grade.py

 from django.core.exceptions import PermissionDenied
+from django.db.models import Q
 
 from graphene_django.types import DjangoObjectType
 from guardian.shortcuts import get_objects_for_user
@@ -45,10 +46,12 @@ class GradeType(SharedSecretObjectType, PermissionsTypeMixin, DjangoFilterMixin,
                 Group.objects.filter(parent_groups__in=owned_groups)
             )
 
+            filters = Q(effort__group__in=groups.values_list("id", flat=True))
+
             if get_site_preferences()["maka__view_own_grades"]:
-                groups = groups.union(info.context.user.person.member_of.all())
+                filters = filters | Q(person=info.context.user.person)
 
-            return super().get_queryset(queryset, info).filter(effort__group__in=groups.values_list("id", flat=True))
+            return super().get_queryset(queryset, info).filter(filters)
 
         raise PermissionDenied()