Don't resolve relationships for history objects (#479)

* Don't resolve relationships for history objects

* Fetch values from instances rather than history objects

* Remove unnecessary `in` check

This will always evaluate to true, anyway

* Don't index current values unnecessarily

* Correct black formatting errors

Author: RealOrangeOne

simple_history/models.py

@@ -9,6 +9,7 @@
 from django.apps import apps
 from django.conf import settings
 from django.contrib import admin
+from django.core.serializers import serialize
 from django.db import models
 from django.db.models import Q
 from django.db.models.fields.proxy import OrderWrt
@@ -24,9 +25,15 @@
 from .manager import HistoryDescriptor
 from .signals import post_create_historical_record, pre_create_historical_record
 
+import json
+
 registered_models = {}
 
 
+def model_to_dict(model):
+    return json.loads(serialize("json", [model]))[0]["fields"]
+
+
 def default_get_user(request, **kwargs):
     try:
         return request.user
@@ -461,18 +468,15 @@ def diff_against(self, old_history):
 
         changes = []
         changed_fields = []
-        instance_fields = [field.name for field in self.instance._meta.fields]
-        old_instance_fields = [
-            field.name for field in old_history.instance._meta.fields
-        ]
-        for field in self._meta.fields:
-            if field.name in instance_fields and field.name in old_instance_fields:
-                old_value = getattr(old_history, field.name, "")
-                new_value = getattr(self, field.name)
+        old_values = model_to_dict(old_history.instance)
+        current_values = model_to_dict(self.instance)
+        for field, new_value in current_values.items():
+            if field in old_values:
+                old_value = old_values[field]
                 if old_value != new_value:
-                    change = ModelChange(field.name, old_value, new_value)
+                    change = ModelChange(field, old_value, new_value)
                     changes.append(change)
-                    changed_fields.append(field.name)
+                    changed_fields.append(field)
 
         return ModelDelta(changes, changed_fields, old_history, self)