php - Codeigniter: Controller issues with Closure table

Question

I'm having issues implementing a controller for a closure table, I'm trying to INSERT a descendant.

Here's what I've done

Controller

public function add() {    
    $this->load->model('home_model', 'closures');

    $node_id = $this->input->post('ancestor');
    $target = array(
        'descendant' => $this->input->post('descendant'),
        'lvl' => $this->input->post('lvl'),
        'id' => $this->input->post('id')
    );  

    $cust_id = $this->closures->add($node_id, $target);
    $this->index();    
    redirect(site_url());
}

Model

 public function add($node_id, $target_id = 0) {
    $sql = 'SELECT ancestor, '.$node_id.', lvl+1
            FROM '.$this->closure_table.'
            WHERE descendant = '.$target_id.'
            UNION ALL SELECT '.$node_id.','.$node_id.',0';
    $query = 'INSERT INTO '.$this->closure_table.'(ancestor, descendant, lvl) ('.$sql.')';
    $result = $this->db->query($query);
    return $result;
}

View

<form name="closures" method="post" action="<?php echo base_url().'index.php/home/add' ?>" >
    <input placeholder="ancestor" type="text" name="ancestor"  required/>

    <input placeholder="descendant" type="text" name="descendant"  required/>
    <input placeholder="lvl" type="text" name="lvl" />
    <input placeholder="id" type="hidden" name="id" value="" />

    <input type="submit" value="Okay" />
</form>

Error messages 1. Array to string conversion 2. Syntax error on the model after UNION.

I'm having some challenges resolving the error messages