[5.1.x] Fixed #35657 -- Made FileField handle db_default values.

Backport of 8deb6bb1fc427762d56646bf7306cbd11fb5bb68 from main.

4 files changed, 27 insertions, 0 deletions

diff --git a/django/db/models/fields/files.py b/django/db/models/fields/files.py
index 7c911f4b23..0716d3599e 100644
--- a/django/db/models/fields/files.py
+++ b/django/db/models/fields/files.py
@@ -9,6 +9,7 @@ from django.core.files.images import ImageFile
 from django.core.files.storage import Storage, default_storage
 from django.core.files.utils import validate_file_name
 from django.db.models import signals
+from django.db.models.expressions import DatabaseDefault
 from django.db.models.fields import Field
 from django.db.models.query_utils import DeferredAttribute
 from django.db.models.utils import AltersData
@@ -197,6 +198,12 @@ class FileDescriptor(DeferredAttribute):
             attr = self.field.attr_class(instance, self.field, file)
             instance.__dict__[self.field.attname] = attr
 
+        # If this value is a DatabaseDefault, initialize the attribute class
+        # for this field with its db_default value.
+        elif isinstance(file, DatabaseDefault):
+            attr = self.field.attr_class(instance, self.field, self.field.db_default)
+            instance.__dict__[self.field.attname] = attr
+
         # Other types of files may be assigned as well, but they need to have
         # the FieldFile interface added to them. Thus, we wrap any other type of
         # File inside a FieldFile (well, the field's attr_class, which is
diff --git a/docs/releases/5.0.8.txt b/docs/releases/5.0.8.txt
index 5cc3faec98..704ecf2c61 100644
--- a/docs/releases/5.0.8.txt
+++ b/docs/releases/5.0.8.txt
@@ -32,3 +32,6 @@ Bugfixes
 * Fixed a bug in Django 5.0 which caused constraint validation to either crash
   or incorrectly raise validation errors for constraints referring to fields
   using ``Field.db_default`` (:ticket:`35638`).
+
+* Fixed a crash in Django 5.0 when saving a model containing a ``FileField``
+  with a ``db_default`` set (:ticket:`35657`).
diff --git a/tests/file_storage/models.py b/tests/file_storage/models.py
index 873c3e176a..cb0207cae9 100644
--- a/tests/file_storage/models.py
+++ b/tests/file_storage/models.py
@@ -72,6 +72,9 @@ class Storage(models.Model):
     default = models.FileField(
         storage=temp_storage, upload_to="tests", default="tests/default.txt"
     )
+    db_default = models.FileField(
+        storage=temp_storage, upload_to="tests", db_default="tests/db_default.txt"
+    )
     empty = models.FileField(storage=temp_storage)
     limited_length = models.FileField(
         storage=temp_storage, upload_to="tests", max_length=20
diff --git a/tests/file_storage/tests.py b/tests/file_storage/tests.py
index 868b18dd2c..c7ca873521 100644
--- a/tests/file_storage/tests.py
+++ b/tests/file_storage/tests.py
@@ -944,6 +944,20 @@ class FileFieldStorageTests(TestCase):
         self.assertEqual(obj.default.read(), b"default content")
         obj.default.close()
 
+    def test_filefield_db_default(self):
+        temp_storage.save("tests/db_default.txt", ContentFile("default content"))
+        obj = Storage.objects.create()
+        self.assertEqual(obj.db_default.name, "tests/db_default.txt")
+        self.assertEqual(obj.db_default.read(), b"default content")
+        obj.db_default.close()
+
+        # File is not deleted, even if there are no more objects using it.
+        obj.delete()
+        s = Storage()
+        self.assertEqual(s.db_default.name, "tests/db_default.txt")
+        self.assertEqual(s.db_default.read(), b"default content")
+        s.db_default.close()
+
     def test_empty_upload_to(self):
         # upload_to can be empty, meaning it does not use subdirectory.
         obj = Storage()