docs: def delete_relation(sender, instance, **kwargs):
      """Delete the Relation object when the last Entity is removed."""
      def process_signal(relation_id):
          """Get the relation and delete it if it has no entities left."""
          try:
              relation = Relation.objects.get(pk=relation_id)
          except Relation.DoesNotExist:
              return
  
          if relation.entities.count() == 0:
              relation.delete()
  
      # Wait for partitions to be recreated.
      transaction.on_commit(lambda: process_signal(instance.relation_id))