From e03e5c751c56db5f4cb99e142c92d7d8db3a5463 Mon Sep 17 00:00:00 2001
From: Simon Charette <charette.s@gmail.com>
Date: Tue, 6 May 2025 13:57:20 -0400
Subject: [PATCH] Fixed #33312 -- Raised explicit exception when copying
 deferred model instances.
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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                  |  1 +
 django/db/models/query_utils.py           |  5 +++--
 docs/topics/db/queries.txt                | 15 +++++++++++++++
 tests/defer_regress/tests.py              | 22 ++++++++++++++++++++++
 tests/model_fields/test_generatedfield.py |  2 +-
 5 files changed, 42 insertions(+), 3 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:
diff --git a/docs/topics/db/queries.txt b/docs/topics/db/queries.txt
index eb5b323abb..bd092f524b 100644
--- a/docs/topics/db/queries.txt
+++ b/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
diff --git a/tests/defer_regress/tests.py b/tests/defer_regress/tests.py
index 1209325f21..c45a503630 100644
--- a/tests/defer_regress/tests.py
+++ b/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])
diff --git a/tests/model_fields/test_generatedfield.py b/tests/model_fields/test_generatedfield.py
index 9bc96a86bb..b6a933451d 100644
--- a/tests/model_fields/test_generatedfield.py
+++ b/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