Codeigniter add a Discount item - php

Question

Hi I have a cart in Codeigniter that works well. I tried to look for a discount/coupon handling but can't.
This is my cart code:
public function addToCart($id){
$this->load->model('products_model');
$products = $this->products_model->getItemDetail($id);
foreach ($products as $product) {
$insert = array(
'id' => $product->id,
'name' => $product->name,
'qty' => '1',
'price' => $product->item_eur,
'image' => $product->image,
'options' => array(
'info' => $product->cart_description,
'qty_description' => $product->qty_description
)
);
$this->cart->insert($insert);
}
$last = end($this->cart->contents());
echo $last['rowid'];
}
I thought of making a Coupon Code read from the DB and insert a discount as an item in the cart:
public function addCoupon(){
$this->load->model('products_model');
$res = $this->products_model->getCoupon($_POST['value']);
if($res){
foreach($res as $result){
$coupon = array(
'id' => $result->id,
'name' => $result->name,
'qty' => '1',
'price' => $result->discount,
'options' => array(
'info' => 'coupon',
'qty_description' => ''
)
);
$this->cart->insert($coupon);
}
echo '1';
}else{
$this->lang->load('products');
echo $this->lang->line('cart_notok');
}
}
Where the DB is
[name:type] (example)
id:int (1)
name: varchar ('Discount of 20€')
code: varchar ('AFD1234')
discount: decimal (-20)
[Edit]
And the model:
public function getCoupon($code){
$results = $this->db->get_where('coupons', array('code' => $code));
return $results->result();
}
The item gets added to the cart but if the 'price' => $result->discount is set to -20 or a percentage it gets set as +20€ and not -20€.
Any way to do this?