TMSS-190: added static method to get a QuerySet for subtasks with no predecessors

SAS/TMSS/src/tmss/tmssapp/models/scheduling.py

@@ -173,6 +173,13 @@ class Subtask(BasicCommon):
                         msg.subject, tobus.exchange, address[0], address[1], single_line_with_single_spaces(msg.content))
             tobus.send(msg)
 
+    @staticmethod
+    def independent_subtasks() -> QuerySet:
+        '''return a QuerySet of all subtasks with no input (i.e. which are "independent" because they have no predecessors)
+           If you want the result, add .all() like so: Subtask.independent_subtasks().all()
+        '''
+        return Subtask.objects.filter(inputs=None)
+
     @property
     def successors(self) -> QuerySet:
         '''return the connect successor subtask(s) as queryset (over which you can perform extended queries, or return via the serializers/viewsets)