TMSS-221: allow empty scheduling units so that tests pass

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

@@ -314,13 +314,21 @@ class SchedulingUnitDraft(NamedCommon):
     def relative_start_time(self) -> datetime.timedelta:
         '''return the earliest relative start time of all tasks of this scheduling unit
         '''
-        return min(self.task_drafts.all(), key=lambda x: x.relative_start_time).relative_start_time
+        task_drafts = list(self.task_drafts.all())
+        if task_drafts:
+            return min(task_drafts, key=lambda x: x.relative_start_time).relative_start_time
+        else:
+            return datetime.timedelta(seconds=0)
 
     @property
     def relative_stop_time(self) -> datetime.timedelta:
         '''return the latest relative stop time of all tasks of this scheduling unit
         '''
-        return max(self.task_drafts.all(), key=lambda x: x.relative_stop_time).relative_stop_time
+        task_drafts = list(self.task_drafts.all())
+        if task_drafts:
+            return max(task_drafts, key=lambda x: x.relative_stop_time).relative_stop_time
+        else:
+            return datetime.timedelta(seconds=0)
 
 
 class SchedulingUnitBlueprint(NamedCommon):
@@ -348,13 +356,21 @@ class SchedulingUnitBlueprint(NamedCommon):
     def relative_start_time(self) -> datetime.timedelta:
         '''return the earliest relative start time of all tasks of this scheduling unit
         '''
-        return min(self.task_blueprints.all(), key=lambda x: x.relative_start_time).relative_start_time
+        task_blueprints = list(self.task_blueprints.all())
+        if task_blueprints:
+            return min(task_blueprints, key=lambda x: x.relative_start_time).relative_start_time
+        else:
+            return datetime.timedelta(seconds=0)
 
     @property
     def relative_stop_time(self) -> datetime.timedelta:
         '''return the latest relative stop time of all tasks of this scheduling unit
         '''
-        return max(self.task_blueprints.all(), key=lambda x: x.relative_stop_time).relative_stop_time
+        task_blueprints = list(self.task_blueprints.all())
+        if task_blueprints:
+            return max(task_blueprints, key=lambda x: x.relative_stop_time).relative_stop_time
+        else:
+            return datetime.timedelta(seconds=0)
 
     @property
     def start_time(self) -> datetime or None: