Added test that reproduces the problem.

auditlog_tests/models.py

@@ -200,6 +200,16 @@ class DateTimeFieldModel(models.Model):
     history = AuditlogHistoryField(delete_related=True)
 
 
+class DecimalFieldModel(models.Model):
+    """
+    A model with a DecimalField, used to test DecimalField changes
+    are detected properly.
+    """
+    decimal = models.DecimalField(max_digits=18, decimal_places=2, default=0)
+
+    history = AuditlogHistoryField(delete_related=True)
+
+
 class ChoicesFieldModel(models.Model):
     """
     A model with a CharField restricted to a set of choices.
@@ -341,6 +351,7 @@ auditlog.register(SimpleExcludeModel, exclude_fields=["text"])
 auditlog.register(SimpleMappingModel, mapping_fields={"sku": "Product No."})
 auditlog.register(AdditionalDataIncludedModel)
 auditlog.register(DateTimeFieldModel)
+auditlog.register(DecimalFieldModel)
 auditlog.register(ChoicesFieldModel)
 auditlog.register(CharfieldTextfieldModel)
 auditlog.register(PostgresArrayFieldModel)

auditlog_tests/tests.py

@@ -6,6 +6,7 @@ import warnings
 from datetime import timezone
 from unittest import mock
 from unittest.mock import patch
+from decimal import Decimal
 
 import freezegun
 from dateutil.tz import gettz
@@ -41,6 +42,7 @@ from auditlog_tests.models import (
     AutoManyRelatedModel,
     CharfieldTextfieldModel,
     ChoicesFieldModel,
+    DecimalFieldModel,
     DateTimeFieldModel,
     JSONModel,
     ManyRelatedModel,
@@ -1196,7 +1198,8 @@ class RegisterModelSettingsTest(TestCase):
 
         self.assertTrue(self.test_auditlog.contains(SimpleExcludeModel))
         self.assertTrue(self.test_auditlog.contains(ChoicesFieldModel))
-        self.assertEqual(len(self.test_auditlog.get_models()), 25)
+        self.assertTrue(self.test_auditlog.contains(DecimalFieldModel))
+        self.assertEqual(len(self.test_auditlog.get_models()), 26)
 
     def test_register_models_register_model_with_attrs(self):
         self.test_auditlog._register_models(
@@ -1463,6 +1466,17 @@ class ChoicesFieldModelTest(TestCase):
         assert "related_models" in history.changes_display_dict
 
 
+class DecimalFieldModelTest(TestCase):
+
+    def setUp(self):
+        self.obj = DecimalFieldModel.objects.create(decimal=0)
+
+    def test_no_changes_detected_after_saving_again(self):
+        self.assertEqual(self.obj.history.count(), 1)
+        self.obj.save()
+        self.assertEqual(self.obj.history.count(), 1)
+
+
 class CharFieldTextFieldModelTest(TestCase):
     def setUp(self):
         self.PLACEHOLDER_LONGCHAR = "s" * 255