Please help with this error

[eluser]aryan_[/eluser]
I have created a model

Code:

$sql = "SELECT * FROM ad_watch WHERE ad_id='{$ad_id}' AND user_id={$user_id} AND datetime BETWEEN {$b_date} AND {$c_date}";

$query = $this->db->query($sql);

if($query->num_rows()==0){
return FALSE;
}
else{
    return TRUE;
}

It throws an error

Code:

A PHP Error was encountered

Severity: Notice

Message: Undefined variable: num_rows

Am I missing something?
Thanks!

[eluser]aryan_[/eluser]
solved! thanks