From 3a58e0a9997eee0a9a5a696659835a14031db8e7 Mon Sep 17 00:00:00 2001
From: Youngkwang Yang <immutable000@gmail.com>
Date: Mon, 9 Jun 2025 22:00:28 +0900
Subject: [PATCH] Fix `get_field_value` field default value handling for Django
 6.0 compatibility (#726)

* 'get_field_value' - improve default value handling

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add Test cases

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
---
 auditlog/diff.py                  | 40 ++++++++++++++++++++-----------
 auditlog_tests/test_app/models.py |  1 +
 auditlog_tests/tests.py           | 27 +++++++++++++++++++++
 3 files changed, 54 insertions(+), 14 deletions(-)

diff --git a/auditlog/diff.py b/auditlog/diff.py
index 5a46731..b8455f1 100644
--- a/auditlog/diff.py
+++ b/auditlog/diff.py
@@ -62,11 +62,31 @@ def get_field_value(obj, field, use_json_for_changes=False):
     :return: The value of the field as a string.
     :rtype: str
     """
+
+    def get_default_value():
+        """
+        Attempts to get the default value for a field from the model's field definition.
+
+        :return: The default value of the field or None
+        """
+        try:
+            model_field = obj._meta.get_field(field.name)
+            default = model_field.default
+            if default is NOT_PROVIDED:
+                return None
+
+            if callable(default):
+                return default()
+
+            return default
+        except AttributeError:
+            return None
+
     try:
         if isinstance(field, DateTimeField):
             # DateTimeFields are timezone-aware, so we need to convert the field
             # to its naive form before we can accurately compare them for changes.
-            value = getattr(obj, field.name, None)
+            value = getattr(obj, field.name)
             try:
                 value = field.to_python(value)
             except TypeError:
@@ -78,30 +98,22 @@ def get_field_value(obj, field, use_json_for_changes=False):
             ):
                 value = django_timezone.make_naive(value, timezone=timezone.utc)
         elif isinstance(field, JSONField):
-            value = field.to_python(getattr(obj, field.name, None))
+            value = field.to_python(getattr(obj, field.name))
             if not use_json_for_changes:
                 try:
                     value = json.dumps(value, sort_keys=True, cls=field.encoder)
                 except TypeError:
                     pass
         elif (field.one_to_one or field.many_to_one) and hasattr(field, "rel_class"):
-            value = smart_str(
-                getattr(obj, field.get_attname(), None), strings_only=True
-            )
+            value = smart_str(getattr(obj, field.get_attname()), strings_only=True)
         else:
-            value = getattr(obj, field.name, None)
-
+            value = getattr(obj, field.name)
             if not use_json_for_changes:
                 value = smart_str(value)
                 if type(value).__name__ == "__proxy__":
                     value = str(value)
-
-    except ObjectDoesNotExist:
-        value = (
-            field.default
-            if getattr(field, "default", NOT_PROVIDED) is not NOT_PROVIDED
-            else None
-        )
+    except (ObjectDoesNotExist, AttributeError):
+        return get_default_value()
 
     return value
 
diff --git a/auditlog_tests/test_app/models.py b/auditlog_tests/test_app/models.py
index 38d6966..cf35cc4 100644
--- a/auditlog_tests/test_app/models.py
+++ b/auditlog_tests/test_app/models.py
@@ -20,6 +20,7 @@ class SimpleModel(models.Model):
     boolean = models.BooleanField(default=False)
     integer = models.IntegerField(blank=True, null=True)
     datetime = models.DateTimeField(auto_now=True)
+    char = models.CharField(null=True, max_length=100, default=lambda: "default value")
 
     history = AuditlogHistoryField(delete_related=True)
 
diff --git a/auditlog_tests/tests.py b/auditlog_tests/tests.py
index c5c9fe8..4be1251 100644
--- a/auditlog_tests/tests.py
+++ b/auditlog_tests/tests.py
@@ -2111,6 +2111,33 @@ class ModelInstanceDiffTest(TestCase):
             msg="ObjectDoesNotExist should be handled",
         )
 
+    def test_field_with_no_default_provided(self):
+        """Field with no default (NOT_PROVIDED) should return None."""
+        first = SimpleModel(integer=1)
+        second = SimpleModel()
+
+        delattr(second, "integer")
+
+        changes = model_instance_diff(first, second)
+        self.assertEqual(
+            changes,
+            {"integer": ("1", "None")},
+            msg="field with no default should return None",
+        )
+
+    def test_field_with_callable_default(self):
+        first = SimpleModel(char="value")
+        second = SimpleModel()
+
+        delattr(second, "char")
+
+        changes = model_instance_diff(first, second)
+        self.assertEqual(
+            changes,
+            {"char": ("value", "default value")},
+            msg="callable default should be handled",
+        )
+
     def test_diff_models_with_json_fields(self):
         first = JSONModel.objects.create(
             json={