Reuse model_to_dict

This not only means comparison is done on primitive values, but as a side effect removes the extra query for base models

simple_history/models.py

@@ -11,6 +11,7 @@
 from django.db import models
 from django.db.models import ManyToManyField
 from django.db.models.fields.proxy import OrderWrt
+from django.forms.models import model_to_dict
 from django.urls import reverse
 from django.utils import timezone
 from django.utils.encoding import smart_str
@@ -603,9 +604,15 @@ def diff_against(self, old_history, excluded_fields=None, included_fields=None):
         changes = []
         changed_fields = []
 
+        old_values = model_to_dict(old_history, fields=fields)
+        current_values = model_to_dict(self, fields=fields)
+
         for field in fields:
-            old_value = getattr(old_history, field)
-            current_value = getattr(self, field)
+            try:
+                old_value = old_values[field]
+                current_value = current_values[field]
+            except KeyError:
+                continue
 
             if old_value != current_value:
                 changes.append(ModelChange(field, old_value, current_value))

simple_history/tests/tests/test_models.py

@@ -646,8 +646,7 @@ def test_history_diff_includes_changed_fields_of_base_model(self):
         r.name = "DonnutsKing"
         r.save()
         new_record, old_record = r.history.all()
-        # Two queries due to base lookup
-        with self.assertNumQueries(2):
+        with self.assertNumQueries(0):
             delta = new_record.diff_against(old_record)
         expected_change = ModelChange("name", "McDonna", "DonnutsKing")
         self.assertEqual(delta.changed_fields, ["name"])