disallow tasks with workflow null

atdb/taskdatabase/models.py

@@ -101,7 +101,7 @@ class Task(models.Model):
     total_processing_time = models.IntegerField(default=0, null=True, blank=True)
 
     # relationships
-    workflow = models.ForeignKey(Workflow, related_name='tasks', on_delete=models.SET_NULL, null=True, blank=True)
+    workflow = models.ForeignKey(Workflow, related_name='tasks', on_delete=models.CASCADE, null=False)
     predecessor = models.ForeignKey('self', related_name='successors', on_delete=models.SET_NULL, null=True, blank=True)
 
     def __str__(self):