[backend] Make parent/child relationship a o2o field

beat/web/backend/migrations/0002_scheduler_addons.py

@@ -142,7 +142,7 @@ class Migration(migrations.Migration):
                 ('end_date', models.DateTimeField(null=True, blank=True)),
                 ('split_errors', models.PositiveIntegerField(default=0)),
                 ('block', models.OneToOneField(related_name='job', on_delete=models.deletion.CASCADE, to='experiments.Block', null=True)),
-                ('parent', models.ForeignKey(related_name='child', to='backend.Job', null=True)),
+                ('parent', models.OneToOneField(related_name='child', to='backend.Job', null=True, on_delete=models.deletion.SET_NULL)),
                 ('result', models.ForeignKey(to='backend.Result', null=True)),
             ],
         ),