Fixed #36847 -- Ensured auto_now_add fields are set on pre_save().

Regression in 94680437a4. Refs #27222.

During INSERT operations, `field.pre_save()` is called to prepare values
for db insertion. The `add` param must be `True` for `auto_now_add`
fields to be populated. The regression commit passed `False`, causing
`auto_now_add` fields to remain `None` when used by other fields, such
as `upload_to` callables.

Thanks Ran Benita for the report.

tests/model_fields/models.py

@@ -262,10 +262,20 @@ class DataModel(models.Model):
 # FileField
 
 
+def upload_to_with_date(instance, filename):
+    return f"{instance.created_at.year}/{filename}"
+
+
 class Document(models.Model):
     myfile = models.FileField(storage=temp_storage, upload_to="unused", unique=True)
 
 
+# See ticket #36847.
+class DocumentWithTimestamp(models.Model):
+    created_at = models.DateTimeField(auto_now_add=True)
+    myfile = models.FileField(storage=temp_storage, upload_to=upload_to_with_date)
+
+
 ###############################################################################
 # ImageField
 

tests/model_fields/test_filefield.py

@@ -13,7 +13,7 @@ from django.db import IntegrityError, models
 from django.test import TestCase, override_settings
 from django.test.utils import isolate_apps
 
-from .models import Document
+from .models import Document, DocumentWithTimestamp
 
 
 class FileFieldTests(TestCase):
@@ -209,3 +209,9 @@ class FileFieldTests(TestCase):
 
         document = MyDocument(myfile="test_file.py")
         self.assertEqual(document.myfile.field.model, MyDocument)
+
+    def test_upload_to_callable_sees_auto_now_add_field_value(self):
+        d = DocumentWithTimestamp(myfile=ContentFile(b"content", name="foo"))
+        d.save()
+        self.assertIsNotNone(d.created_at)
+        self.assertIs(d.myfile.name.startswith(f"{d.created_at.year}/foo"), True)