Allows nullable MonitorField to be populated on creation

CHANGES.rst

@@ -6,6 +6,7 @@ To be released
 - Add support for `Python 3.13` (GH-#628)
 - Add formal support for `Django 5.2` (GH-#641)
 - Drop support for older versions than `Django 4.2`
+- Fixes MonitorField not updating on creation with `when` and `null=True`
 
 5.0.0 (2024-09-01)
 ------------------

model_utils/fields.py

@@ -148,10 +148,14 @@ def pre_save(self, model_instance: models.Model, add: bool) -> Any:
         value = now()
         previous = getattr(model_instance, self.monitor_attname, None)
         current = self.get_monitored_value(model_instance)
-        if previous != current:
-            if self.when is None or current in self.when:
-                setattr(model_instance, self.attname, value)
-                self._save_initial(model_instance.__class__, model_instance)
+
+        should_update = previous != current
+        if self.when is not None:
+            should_update = (should_update or add) and current in self.when
+
+        if should_update:
+            setattr(model_instance, self.attname, value)
+            self._save_initial(model_instance.__class__, model_instance)
         return super().pre_save(model_instance, add)
 
     def deconstruct(self) -> tuple[str, str, Sequence[Any], dict[str, Any]]:

tests/models.py

@@ -123,6 +123,11 @@ class MonitorWhenEmpty(models.Model):
     name_changed = MonitorField(monitor="name", when=[])
 
 
+class MonitorWhenNullable(models.Model):
+    name = models.CharField(max_length=25)
+    name_changed = MonitorField(monitor="name", when=["Jose"], null=True)
+
+
 class DoubleMonitored(models.Model):
     name = models.CharField(max_length=25)
     name_changed = MonitorField(monitor="name")

tests/test_fields/test_monitor_field.py

@@ -6,7 +6,13 @@
 from django.test import TestCase
 
 from model_utils.fields import MonitorField
-from tests.models import DoubleMonitored, Monitored, MonitorWhen, MonitorWhenEmpty
+from tests.models import (
+    DoubleMonitored,
+    Monitored,
+    MonitorWhen,
+    MonitorWhenEmpty,
+    MonitorWhenNullable,
+)
 
 
 class MonitorFieldTests(TestCase):
@@ -85,6 +91,23 @@ def test_double_save(self) -> None:
         self.assertEqual(self.instance.name_changed, changed)
 
 
+class MonitorWhenNullableFieldTests(TestCase):
+    """
+    Will record changes only when name is 'Jose'
+    """
+    def test_created_Jose(self) -> None:
+        with time_machine.travel(datetime(2016, 1, 1, 12, 0, 0, tzinfo=timezone.utc)):
+            instance = MonitorWhenNullable(name='Jose')
+            instance.save()
+        self.assertEqual(instance.name_changed, datetime(2016, 1, 1, 12, 0, 0, tzinfo=timezone.utc))
+
+    def test_created_Charlie(self) -> None:
+        with time_machine.travel(datetime(2016, 1, 1, 12, 0, 0, tzinfo=timezone.utc)):
+            instance = MonitorWhenNullable(name='Charlie')
+            instance.save()
+        self.assertIsNone(instance.name_changed)
+
+
 class MonitorWhenEmptyFieldTests(TestCase):
     """
     Monitor should never be updated id when is an empty list.