Fixed #33312 -- Raised explicit exception when copying deferred model instances.

Previously save() would crash with an attempted forced update message, and both
save(force_insert=True) and bulk_create() would crash with DoesNotExist errors
trying to retrieve rows with an empty primary key (id IS NULL).

Implementing deferred field model instance copying might be doable in certain
cases (e.g. when all the deferred fields are db generated) but that's not
trivial to implement in a backward compatible way.

Thanks Adam Sołtysik for the report and test and Clifford for the review.

2 files changed, 4 insertions, 2 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 72ba0471a4..d4559e0693 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -859,6 +859,7 @@ class Model(AltersData, metaclass=ModelBase):
             not force_insert
             and deferred_non_generated_fields
             and using == self._state.db
+            and self._is_pk_set()
         ):
             field_names = set()
             pk_fields = self._meta.pk_fields
diff --git a/django/db/models/query_utils.py b/django/db/models/query_utils.py
index f0ae810f47..3e644a3c26 100644
--- a/django/db/models/query_utils.py
+++ b/django/db/models/query_utils.py
@@ -220,9 +220,10 @@ class DeferredAttribute:
             # might be able to reuse the already loaded value. Refs #18343.
             val = self._check_parent_chain(instance)
             if val is None:
-                if not instance._is_pk_set() and self.field.generated:
+                if not instance._is_pk_set():
                     raise AttributeError(
-                        "Cannot read a generated field from an unsaved model."
+                        f"Cannot retrieve deferred field {field_name!r} "
+                        "from an unsaved model."
                     )
                 instance.refresh_from_db(fields=[field_name])
             else: