refactor(history): move get_diff method to APISHistoryTableBase

Having the `get_diff` method as part of the historical record itself has
the advantage, that it can be used from different parts of the code.

apis_core/history/models.py

@@ -96,6 +96,24 @@ def set_version_tag(self, tag: str, include_triples: bool = True):
                 triple.save()
         self.save()
 
+    def get_diff(self, other_version=None):
+        if self.history_type == "-":
+            return None
+        version = other_version or self.prev_record
+        if version:
+            delta = self.diff_against(version)
+        else:
+            delta = self.diff_against(self.__class__())
+        changes = list(
+            filter(
+                lambda x: (x.new != "" or x.old is not None)
+                and x.field != "id"
+                and not x.field.endswith("_ptr"),
+                delta.changes,
+            )
+        )
+        return sorted(changes, key=lambda change: change.field)
+
 
 class VersionMixin(models.Model):
     history = APISHistoricalRecords(

apis_core/history/serializers.py

@@ -59,19 +59,9 @@ class HistoryLogSerializer(serializers.Serializer):
     def get_diff(self, obj):
         if obj.history_type == "-":
             return None
-        if obj.prev_record is None:
-            diff = obj.diff_against(obj.__class__())
-        else:
-            diff = obj.diff_against(obj.prev_record)
         changed_fields = []
         changes = []
-        for change in diff.changes:
-            if (
-                (change.new == "" and change.old is None)
-                or change.field == "id"
-                or change.field.endswith("_ptr")
-            ):
-                continue
+        for change in obj.get_diff():
             changed_fields.append(change.field)
             changes.append(ModelChangeSerializer(change, obj).data)
         return {"changed_fields": changed_fields, "changes": changes}