Removing one item from opencart cart removes everything - opencart

Question

My client is using an Opencart v2.1.0.2. The problem is when I remove a single item from the cart, all other items also gets removed. Yes, it's using a custom template and here's the line that calls for the delete function:
<td class="text-center"><button type="button" onclick="cart.remove('<?php echo $product['cart_id']; ?>');" title="<?php echo $button_remove; ?>" class="btn btn-danger btn-xs"><i class="fa fa-times"></i></button></td>
I also need to find out what js file is being called when this button is clicked. If it'll help, this I think this is the function from the cart.php library that opencart runs when deleting an item from the cart:
public function remove($cart_id) {
$this->db->query("DELETE FROM " . DB_PREFIX . "cart WHERE cart_id = '" . (int)$cart_id . "' AND customer_id = '" . (int)$this->customer->getId() . "' AND session_id = '" . $this->db->escape($this->session->getId()) . "'");
}

Answer 1

DELETE FROM
removes everything. It's missing the product id of what it's trying to remove.
WHERE only specifies: cart_id, customer_id and session_id - so this query removes everything matching that criteria. It doesn't hone in on a specific product, so it'll take them all.
Without knowing what variable it uses for the product ID, a proper query would be something like:
public function remove($cart_id) {
$this->db->query("DELETE '".(int)$this->request->post['product_id']."' FROM " . DB_PREFIX . "cart WHERE cart_id = '" . (int)$cart_id . "' AND customer_id = '" . (int)$this->customer->getId() . "' AND session_id = '" . $this->db->escape($this->session->getId()) . "'");
}
(int)$this->request->get['product_id'] being the $_GET request for most product id calls.
Another instance could be:
(int)$this->request->post['product_id'] for a $_POST call
The template may also do something like $product_id so check what variable its using if the post/get doesn't work.
Once you got the query tightened up, the javascript button only specifies a cart id (not a product). Get your product ID in there if you want my example to work. You'll need to pass both the product_id and cart_id; else, without cart_id, if other shoppers have the same item at the same time, one person removing it would remove for them all.