MySQLi num_rows总是返回零

Question

这可能是一个非常愚蠢的错误，但我找不到它。任何帮助，将不胜感激。尝试获取select查询中的行数。即使应该返回行，它也始终返回零。

$DESC_id = $_POST['desc_id']; //array       
$DESC = $_POST['desc']; //array
$DESC_hr = $_POST['desc_hr']; //array
$DESC_rt = $_POST['desc_rt']; //array
$DESC_amt = $_POST['desc_amt']; //array 
...

if(!empty($DESC)){

        foreach ($DESC as $i => $b) {   
            //SQL Injection Prevention  

            $v_DESC_id[$i]="'" . $conn->real_escape_string($DESC_id[$i]) . "'";
            $v_DESC[$i]="'" . $conn->real_escape_string($DESC[$i]) . "'";
            $v_DESC_hr[$i]="'" . $conn->real_escape_string($DESC_hr[$i]) . "'";
            $v_DESC_rt[$i]="'" . $conn->real_escape_string($DESC_rt[$i]) . "'"; 
            $v_DESC_amt[$i]="'" . $conn->real_escape_string($DESC_amt[$i]) . "'";    


    $sql = "SELECT * FROM estimate_desc WHERE id = ?";
    $stmt = $conn->prepare($sql);

    if($stmt ===false){
  trigger_error('Wrong SQL: ' .$sql. ' Error: ' . $conn->error, E_USER_ERROR);
}

$stmt ->bind_param('i', $v_DESC_id[$i]);
$stmt->execute(); 
$result = $stmt->get_result();              

//this prints out the sql query which I've double checked in myadmin and it returns a 
echo 'SELECT * FROM estimate_desc WHERE id = '.$v_DESC_id[$i];

$q_row_num =  $result->num_rows;
echo '<br>row number is '.$q_row_num;
$stmt->close();

...
}