Error query php / codeigniter

I am using the code below to query the database, but it is giving the following error:

  

PHP Error was encountered   Severity: Notice   Message: Undefined property: Admin :: $ db   Filename: core / Model.php   Line Number: 51

     

Fatal error: Call to a member function query () on a non-object in /home/*/public_html/application/models/admin_model.php on line 4 **

Code

class Admin_model extends CI_Model {
   public function carregar_hoteis(){
        $hoteis = $this->db->query("SELECT * FROM tabela");
        $array=array();
        foreach($hoteis->result() as $row)
        {
             $array['hoteis'][]=$row;
        }
        echo json_encode($array);
   }
}