ThinkPHP6手动分页：如何处理查询条件中缺少库存字段的情况？

list($where, $alias, $limit, $order) = $this->queryBuilder();$res = $this->model    ->field($this->indexField)    ->withJoin($this->withJoinTable, $this->withJoinType)    ->alias($alias)    ->where($where)    ->order($order)    ->paginate($limit)    ->each(function ($item, $key) {        $product_in = Db::name('product_in')->where('product_id', $item['id'])->sum('quantity'); //该物品的入库数量        $product_out = Db::name('product_out')->where('product_id', $item['id'])->sum('quantity'); //该物品的出库数量        $item['stock'] = $product_in - $product_out; //这里是库存计算    });$this->success('', [    'list'   => $res->items(),    'total'  => $res->total(),    'remark' => get_route_remark(),]);