Problem when updating with ID of another table in CodeIgniter

These are the 3 tables I have:

ci_comments : comment_id, email, website, body.

ci_relationship : id, comment_id, user_id.

ci_users : user_id, username, email, biography.

The problem is that I'm trying to get data from two tables, one is that of ci_comments that shows all the content of the comments and the other table to get the user_id I mention.

I have this in my edit method in the Comments controller:

// Field Rules
        $this->form_validation->set_rules('user_id', 'User ID', 'trim|required');
        $this->form_validation->set_rules('body', 'body', 'trim|required');
        $this->form_validation->set_rules('status', 'Status', 'required');

        if ($this->form_validation->run() == FALSE) {

            $data['item'] = $this->Comments_model->get($id);

            // AQUI ES DONDE NECESITO AYUDA - No estoy ni seguro si es que debería pasar una variable o no //
// Recuerden que el user_id debe de venir de la table ci_relationship
            $data['commentator'] = $this->Comments_model->get_userID($userID);

            // /AQUI ES DONDE NECESITO AYUDA //

            // Select Comment Author ID
            $user_options = array();
            $user_options[0] = 'Select Comment Author ID';

            $user_list = $this->User_model->get_list();
            foreach($user_list as $username){
                $user_options[$username->id] = $username->username;
            }

            $data['user_options'] = $user_options;

            // Load template
            $this->template->load('admin', 'default', 'comments/edit', $data);

        }

Here is the function of the Model that I use to get the content of the current comment:

public function get($id)
    {
        $this->db->where('id', $id);
        $query = $this->db->get('ci_comments');
        return $query->row();
    }

And here is the function that I am trying to create to get the user_id of the comment:

public function get_userID($userID){
    $this->db->select('*');
    $this->db->where('user_id', $userID);
    $this->db->where('type', $this->type);
    $query = $this->db->get('ci_relationship');
    return $query->row();
}

Basically I want to show a dropdown where it is supposed to show all the users that have made a comment but not before showing the specific user of the comment:

<div class="form-group">
        <?php echo form_label('Author ID', 'user_id');  ?>
        <?php echo form_dropdown('user_id', $user_options, $commentator->user_id, array('class' => 'form-control')); ?>
    </div>

NOTE: The user_id should be shown in the third parameter $ commentator-> user_id but it shows me null, now if I change it to $ commentator ['user_id] it continues with the same.

I hope I could have explained myself well, otherwise I would not mind editing the question to give more details.