Subtraction on database value

Question

I'm having a problem in subtracting the amount inputted by the user to the current stock count. here is my code.

here is some of my code. I have a link in index of ItemReg that will open the withdraw.cshtml once click then, when the user put a value on the input box the value will be deducted to the database.

Controller

public IActionResult Withdraw(int? id, int quantity)
     {
          return View(_context.ItemRegs.Find(id));
     }

public async Task<IActionResult> Withdraw([Bind("ItemId,Qty")] ItemReg itemReg,int? id, int quantity) 
        {
            if (ModelState.IsValid) {

                var itemreg = _context.ItemRegs.Find(id);
                itemreg.Qty = itemreg.Qty - quantity;
                _context.Update(itemReg);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }

            return View(itemReg);
        }

withdraw.cshtml

@model Intranet.Models.ItemReg

@{
    ViewData["Title"] = "View";
}

<h4>ItemReg</h4>
<hr />
<div class="row">
    <div class="col-md-4">
        <form asp-action="View">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <input type="hidden" asp-for="ItemId" />
            <div class="form-group">
                <label asp-for="Qty" class="control-label"></label>
                <input name="quantity" asp-for="Qty" class="form-control"/>
                <span asp-validation-for="Qty" class="text-danger"></span>
            </div>
        <div class="form-group">
            <input type="submit" value="Save" class="btn btn-primary" />
        </div>
        </form>
    </div>
</div>

<div>
    <a asp-action="Index">Back to List</a>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

Answer 1

You are using id as the source of your Find(id) . Try passing itemReg.ItemId in your Find(itemReg.ItemId) as your hidden field is ItemId

<input type="hidden" asp-for="ItemId" />

which is binding in the model is not used, try using your model.Id

var itemreg = _context.ItemRegs.Find(itemReg.ItemId);
itemreg.Qty = itemreg.Qty - quantity;
_context.Update(itemReg);

Answer 2

Im not sure if this is a good practice but this answers my question.

public async Task<IActionResult> Withdraw([Bind("ItemId,Qty")] ItemReg itemReg,int quantity) 
        {
            ViewBag.qty = quantity;
            //if (ModelState.IsValid) {
                var itemreg = _context.ItemRegs.Find(itemReg.ItemId);
                itemreg.Qty = itemreg.Qty - ViewBag.qty;
                itemReg.UserName = ViewBag.DisplayName;
                itemreg.UserIP = HttpContext.Connection.RemoteIpAddress.ToString();
                itemreg.UserDate = DateTime.Now.ToString("MM/dd/yyyy");
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            //}
            //return View(itemReg);
        }

I commented the if statement then add the int quantity in the argument.