致命错误：在非对象上调用成员函数fetch_assoc（）

i have an error in my code while using function fetch_assoc

<?php
    include_once('functions.php'); 

    $book_selected=$_POST['book_selected']; 
    $database = new mysqli('127.0.0.1', 'user', 'user', 'library'); 
    $query = 'SELECT * FROM book WHERE title=\'' . $book_selected . '\''; 
    $result_set = $database->query($query); 
    $row = $result_set->fetch_assoc(); 
    var_dump($row);
    $query = 'UPDATE poll_result SET num_poll = num_poll + 1 WHERE id='. $row['id']; 
    $database->query($query); 
    redirect('show_polling.php');
?>

and I got the error :

Fatal error: Call to a member function fetch_assoc() on a non-object

I don't know how to fix it.

Thanks for advice

I'm pretty sure you have an error in your query.

Try this:

<?php
include_once('functions.php'); 

$database = new mysqli('127.0.0.1', 'user', 'user', 'library'); 

if ($database->connect_error) {
    die('Connect Error: ' . $database->connect_error);
}

$book_selected =  $database->real_escape_string($_POST['book_selected']);
$query = 'SELECT * FROM book WHERE title=\'' . $book_selected . '\''; 

$result_set = $database->query($query); 

if (!$result_set) {
    die(sprintf("Error: %s", $database->error));
}

$row = $result_set->fetch_assoc(); 

var_dump($row);

$query = 'UPDATE poll_result SET num_poll = num_poll + 1 WHERE id='. $row['id']; 
$database->query($query); 

redirect('show_polling.php');
?>

In addition to that, you should not pass a variable directly in a query (possibility of SQL injections). In the example above, I escaped the query with mysqli::real_escape_string, but in common, it's better to prepare the query: http://php.net/manual/en/mysqli.prepare.php