# history_tracking/mixins.py


class HistoricalChangeMixin:
    @property
    def prev_record(self):
        """Get the previous record for this instance"""
        try:
            return type(self).objects.filter(
                history_date__lt=self.history_date,
                id=self.id
            ).order_by('-history_date').first()
        except (AttributeError, TypeError):
            return None

    @property
    def diff_against_previous(self):
        prev_record = self.prev_record
        if not prev_record:
            return {}

        changes = {}
        for field in self.__dict__:
            if field not in [
                "history_date",
                "history_id",
                "history_type",
                "history_user_id",
                "history_change_reason",
                "history_type",
            ] and not field.startswith("_"):
                try:
                    old_value = getattr(prev_record, field)
                    new_value = getattr(self, field)
                    if old_value != new_value:
                        changes[field] = {"old": old_value, "new": new_value}
                except AttributeError:
                    continue
        return changes