fix delete

prompt/models.py

@@ -1,7 +1,5 @@
 import uuid
 from django.db import models
-from django.db.models.signals import pre_delete
-from django.dispatch import receiver
 from django_extensions.db.models import TimeStampedModel
 from account.models import User
 from task.models import Task
@@ -38,7 +36,7 @@ class Message(models.Model):
         "submission.Submission",
         null=True,
         blank=True,
-        on_delete=models.SET_NULL,
+        on_delete=models.CASCADE,
         related_name="source_message",
     )
 
@@ -47,10 +45,3 @@ class Message(models.Model):
 
     def __str__(self):
         return f"[{self.role}] {self.content[:50]}"
-
-
-@receiver(pre_delete, sender=Message)
-def delete_linked_submission(sender, instance, **kwargs):
-    if instance.submission_id:
-        from submission.models import Submission
-        Submission.objects.filter(id=instance.submission_id).delete()