Codeigniter, Model, PHP: Check if user already exists in database

gistfile1

public function user_exists($user_id)
{	
	$this->db->where('user_id', $user_id);
	$query = $this->db->get('sometablenamehere');
	if($query->num_rows >= 1)
	{
		//if query finds one row relating to this user then execute code accordingly here
	}
}