Comparing two values from two different tables using nested foreach loop - php

Question

I have a table of item and a table cart. What I'm trying to do is fetch all instances of item and cart then I put them in the loop such that whenever the item's item_id matches the cart's item_id, I'll subtract cart's item_quantity value from item's number_of_units. The result is then used to update item's number_of_items. There's seems to be a problem with my nested foreach loop.
Here's a function in my controller
public function checkout(){
$this->load->model('HomeModel');
$x['item'] = $this->HomeModel->displayAllItems();
$y['cart'] = $this->HomeModel->displayAllCartItems();
foreach($x['item'] as $item){
foreach($y['cart'] as $cart){
if(($item->item_id) == ($cart->item_id)){
$temp1 = $item->number_of_units;
$temp2 = $cart->item_quantity;
$temp = $temp1 - $temp2;
$z = array(
'number_of_units' => $temp,
);
$this->HomeModel->updateItem($z, $item->item_id);
}
}
}
}
Here's some of my model functions:
public function displayAllCartItems()
{
$query = $this->db->select('*')->from('cart')->get();
return $query->result();
}
public function displayAllItems()
{
$query = $this->db->select('*')->from('item')->get();
return $query->result();
}
public function updateItem($data, $id)
{
$this->db->where('item_name', $id);
$query = $this->db->update('item', $data);
}