Admin::isSortableColumn PHP Method

    public function isSortableColumn($attribute)
    {
        if ($this->isSubList) {
            return false;
        }
        if (in_array('display_order', $this->listFields, true)) {
            return false;
        }
        if (strpos($attribute, 'has_') === 0) {
            return false;
        }
        if (in_array($attribute, $this->unsortableColumns, true)) {
            return false;
        }
        return true;
    }