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.

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

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:

docs/topics/db/queries.txt

@@ -1590,6 +1590,21 @@ For example, assuming ``entry`` is already duplicated as above::
     detail.entry = entry
     detail.save()
 
+Note that it is not possible to copy instances of models with deferred fields
+using this pattern unless values are assigned to them:
+
+.. code-block:: pycon
+
+    >>> blog = Blog.objects.defer("name")[0]
+    >>> blog.pk = None
+    >>> blog._state.adding = True
+    >>> blog.save()
+    Traceback (most recent call last):
+        ...
+    AttributeError: Cannot retrieve deferred field 'name' from an unsaved model.
+    >>> blog.name = "Another Blog"
+    >>> blog.save()
+
 .. _topics-db-queries-update:
 
 Updating multiple objects at once

tests/defer_regress/tests.py

@@ -366,3 +366,25 @@ class DeferDeletionSignalsTests(TestCase):
         Proxy.objects.only("value").get(pk=self.item_pk).delete()
         self.assertEqual(self.pre_delete_senders, [Proxy])
         self.assertEqual(self.post_delete_senders, [Proxy])
+
+
+class DeferCopyInstanceTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        SimpleItem.objects.create(name="test", value=42)
+        cls.deferred_item = SimpleItem.objects.defer("value").first()
+        cls.deferred_item.pk = None
+        cls.deferred_item._state.adding = True
+        cls.expected_msg = (
+            "Cannot retrieve deferred field 'value' from an unsaved model."
+        )
+
+    def test_save(self):
+        with self.assertRaisesMessage(AttributeError, self.expected_msg):
+            self.deferred_item.save(force_insert=True)
+        with self.assertRaisesMessage(AttributeError, self.expected_msg):
+            self.deferred_item.save()
+
+    def test_bulk_create(self):
+        with self.assertRaisesMessage(AttributeError, self.expected_msg):
+            SimpleItem.objects.bulk_create([self.deferred_item])

tests/model_fields/test_generatedfield.py

@@ -180,7 +180,7 @@ class GeneratedFieldTestMixin:
 
     def test_unsaved_error(self):
         m = self.base_model(a=1, b=2)
-        msg = "Cannot read a generated field from an unsaved model."
+        msg = "Cannot retrieve deferred field 'field' from an unsaved model."
         with self.assertRaisesMessage(AttributeError, msg):
             m.field