类CI_DB_mysqli_result的对象无法转换为字符串？

hello i'm new in CodeIgniter, I've tried to read the documentation of CI but I still can't solve my problem, maybe someone here can help fix my problem. here my code:

$subdomain = $this->config->item('subdomain_name');
$instansi_id = $this->db->query("SELECT instansi_id FROM instansi WHERE subdomain = '$subdomain'");

$data = array(
'promosi'     => $this->db->query("SELECT * FROM gallery WHERE gallery_status = 1 AND instansi_id = $instansi_id ORDER BY gallery_created  DESC")->result(), 
'slide'       => $this->frontend->getSlide(),
),

i get an error like this: Object of class CI_DB_mysqli_result could not be converted to string

Maybe someone here can help to solve my problem ? Thanks.

In 2nd line you're getting query results which is an object [CI_DB_mysqli_result], but then you're just assuming it's a string in your next query.

$instansi_id = $this->db->query("SELECT instansi_id FROM instansi WHERE subdomain = '$subdomain'")->row()->instansi_id;

This should fix it.

If not, just

var_dump($instansi_id)

To see what's in it and proceed from there.