在codeigniter中使用all_to_array返回null值

Question

我遇到了all_to_array的问题。它返回一个空值。

 $object_self = new self();

            $response = $object_self->
    select('DATE(`created_at`) as `day`')->select_min('paid_amount','min_amount')
->get('orders');

             $query_object = $response->all_to_array(array(
                                                                'day',
                                                                'min_amount',

                                                   ));

$ query_object是一个空值，尽管查询是正确的。