Fixed #13296 -- Fixed ordering by Options.order_with_respect_to after deleting objects.

Thanks Simon Meers for the original patch.

django/db/models/base.py

@@ -15,13 +15,16 @@ from django.db import (
     DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY, DatabaseError, connection,
     connections, router, transaction,
 )
-from django.db.models import NOT_PROVIDED
+from django.db.models import (
+    NOT_PROVIDED, ExpressionWrapper, IntegerField, Max, Value,
+)
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.constraints import CheckConstraint, UniqueConstraint
 from django.db.models.deletion import CASCADE, Collector
 from django.db.models.fields.related import (
     ForeignObjectRel, OneToOneField, lazy_related_operation, resolve_relation,
 )
+from django.db.models.functions import Coalesce
 from django.db.models.manager import Manager
 from django.db.models.options import Options
 from django.db.models.query import Q
@@ -869,9 +872,12 @@ class Model(metaclass=ModelBase):
                 # autopopulate the _order field
                 field = meta.order_with_respect_to
                 filter_args = field.get_filter_kwargs_for_object(self)
-                order_value = cls._base_manager.using(using).filter(**filter_args).count()
-                self._order = order_value
-
+                self._order = cls._base_manager.using(using).filter(**filter_args).aggregate(
+                    _order__max=Coalesce(
+                        ExpressionWrapper(Max('_order') + Value(1), output_field=IntegerField()),
+                        Value(0),
+                    ),
+                )['_order__max']
             fields = meta.local_concrete_fields
             if not pk_set:
                 fields = [f for f in fields if f is not meta.auto_field]