Using CONCAT() in active record

[eluser]AIM-andrew[/eluser]
I have a similar problem but I'm struggling with syntax.
This is my predecessor's code (which works-if the wrong way now):

Code:

public function addToUserId($email, $new_to_user_id) {
     $this->db->select('username');
     $this->db->from('fa_user');
     $this->db->from('user');
     $this->db->where('fa_user_id = id');
     $this->db->where('user_id', $new_to_user_id);
     $results = $this->db->get()->result();

     $email_to = $email->getTo();
     $email_to[] = $results[0]->username;
}

This is the code I wish to implement:

Code:

public function addToUserId($email, $new_to_user_id) {
     // SFPD uses [email protected] format
     // $this->db->select('username');
     $this->db->select('CONCAT (firstname, \'.\', lastname) AS full_name');
     $this->db->from('fa_user');
     $this->db->from('user');
     $this->db->where('fa_user_id = id');
     $this->db->where('user_id', $new_to_user_id);
     $results = $this->db->get()->result();

     $email_to = $email->getTo();
     // $email_to[] = $results[0]->username;
     $email_to[] = $results[0]->full_name;
}

This method retrieved the username based on the user_id currently logged in and appends a static domain (another method). Now we want to retrieve the firstname(dot)lastname combination as per the new email address format.

Any advise?
thx in advance