  thinkphp 5.1 模板中三元比较失效

<select name="tra_num">
        <option value="" {$num_tra=="%"?'selected':''}>全部</option>
        {volist name="list_tra" id="arr_tra" empty="没有文章"}
        <option value="{$arr_tra.id}" { {$num_tra} == {$arr_tra.id} ?'selected':''}>{$arr_tra.id}</option>
        {/volist}
      </select>

这样写双花括号无效  从调试程序中看是以下状况

<select name="tra_num">
        <option value="" >全部</option>
                <option value="1" { (7 == 1 ?'selected':'')}>1</option>
                <option value="2" { (7 == 2 ?'selected':'')}>2</option>
                <option value="4" { (7 == 4 ?'selected':'')}>4</option>
                <option value="5" { (7 == 5 ?'selected':'')}>5</option>
                <option value="6" { (7 == 6 ?'selected':'')}>6</option>
                <option value="7" { (7 == 7 ?'selected':'')}>7</option>
              </select>
    </div>