判断是否购买，代码为什么不生效

  {if $member.uid}
                            {if dr_is_buy($ci->module['field']['pays']['id'], $id, $member.uid, '指定sku字符串值/也可以留空')}
                            已购买
                            {else}
                     <a style="margin-top: 10px;" href="javascript:add_buy();" class="buy btn">立即购买</a>
    {/if}