private function oldPrimaryKey()
    {
        if ($this->oldPrimaryKey === null) {
            if ($this->isPrimaryKeyDifferent()) {
                $oldKey = $this->climate()->arguments->get('old_key');
            } else {
                $oldKey = $this->targetModel()->key();
            }

            $this->oldPrimaryKey = $oldKey;
        }

        return $this->oldPrimaryKey;
    }