Laravel - How to update data calculate by it's column - php

Question

How to update data with calculating by it's column
I have table item
+---------+-------+-------+-----------+
| item_id | stock | price | name |
+---------+-------+-------+-----------+
| 1 | 10 | 9.4 | Paper |
+---------+-------+-------+-----------+
| 2 | 10 | 12.8 | Pencil |
+---------+-------+-------+-----------+
| 3 | 10 | 99.9 | Note Book |
+---------+-------+-------+-----------+
and then I wanna update the sock by calculating stock column
stock - $request->qty
If I do manualy base query
update items set stock = stock - 3 where item_id = 1
it's working, but when I use laravel eloquent the stock become 0 (zero), I dunno why,
here is my controller
public function saveOrder(Request $request) {
$item = new Item;
$item->where('item_id', '=', $request->item_id)->update(['stock' => 'stock - '. $request->qty]);
return redirect('order');
}
any solution? Thanks for answer, sorry for bad english :)

Answer 1

Try this one:
$item = new Item;
$item->where('item_id', '=', $request->item_id)->decrement('stock', $request->qty);

Answer 2

You could try something like this:
$item = Item::where('item_id', '=', $request->item_id)->first();
$item->stock = (int)$item->stock - (int)$request->qty;
$item->save();
Alternatively you could try to use DB::raw expression.

Answer 3

This should do the trick for you
public function saveOrder(Request $request) {
Item::where('item_id', $request->item_id)
->decrement('stock', $request->qty);
return redirect('order');
}

Answer 4

public function saveOrder(Request $request){
$stockdetails = \DB::table('tablename')->where('itemid',$request->itemid)->first(['stock as stock']);
//update query
$update= \DB::table('tablename')->update(['stock'=>($stockdetails->stock)-($request->quantity)])->where(itemid',$request->itemid);
if($update){
//sucecss
}else{
//Fail
}