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

Backport of 8deb6bb from main.

Author: sarahboyce

django/db/models/fields/files.py

@@ -8,6 +8,7 @@
 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
@@ -192,6 +193,12 @@ def __get__(self, instance, cls=None):
             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

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`).

tests/file_storage/models.py

@@ -72,6 +72,9 @@ def pathlib_upload_to(self, filename):
     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

tests/file_storage/tests.py

@@ -895,6 +895,20 @@ def test_filefield_default(self):
         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()