asp.net core duplicate entry catch error

Question

I want to prevent duplicate entry on post, which works well when there is no parameter for OnGet. Code below

 

public void OnGet()
        {

        }


        public async Task<IActionResult> OnPostAsync()
        {
            if (!ModelState.IsValid)
            {
                return Page();
            }

            var newWOAL = new WOAssetLocation();
            if (await TryUpdateModelAsync(newWOAL, "WOAssetLocation", s => s.AssetLocation))
            {
                var check = await _context.WOAssetLocations.Where(s => s.AssetLocation == newWOAL.AssetLocation).CountAsync();
                if (check > 0)
                {
                    ModelState.AddModelError(string.Empty, "Asset Location already exists");
                    return Page();
                }
                if (check == 0)
                {
                    _context.WOAssetLocations.Add(newWOAL);
                    await _context.SaveChangesAsync();
                    return RedirectToPage("/WO/Location/Index");
                }
            }

            return null;
        }

 The duplicate is captured and I have error mesage displayed on page. However, when I have OnGet with parameter is not working

 

public async Task<IActionResult> OnGetAsync(string id)
        {
            if (id == null)
            {
                return NotFound();
            }
            var userId = User.FindFirstValue(ClaimTypes.NameIdentifier);
            var idorg = await _context.UsersData.Where(u => u.Id == userId).Select(u => u.OrgID).SingleAsync();
            OrgID = idorg;
            WOAssetLocation = await _context.WOAssetLocations.Where(m => m.WOALId == id).FirstOrDefaultAsync();

            if (WOAssetLocation == null)
            {
                return NotFound();
            }
            return Page();
        }
        public async Task<IActionResult> OnPostAsync()
        {
            if (!ModelState.IsValid)
            {
                return Page();
            }
            var userId = User.FindFirstValue(ClaimTypes.NameIdentifier);
            var idorg = await _context.UsersData.Where(u => u.Id == userId).Select(u => u.OrgID).SingleAsync();
            var newWOAA = new WOAssetAsset();
            if (await TryUpdateModelAsync(newWOAA, "WOAssetAsset",
                s => s.WOALId,
                s => s.OrgID,
                s => s.AssetID,
                s => s.AssetName,
                s => s.AssetManufacturer))
            {
                var check = await _context.WOAssetAssets.Where(s => s.AssetID == newWOAA.AssetID).CountAsync();
                if (check > 0)
                {
                    ModelState.AddModelError(string.Empty, "Asset ID already exists");
                    return Page();
                }
                if (check == 0)
                {
                    _context.WOAssetAssets.Add(newWOAA);
                    await _context.SaveChangesAsync();
                    return RedirectToPage("/WO/Asset/Index", new { id = newWOAA.WOALId });
                }
            }

            return null;
        }

 the application is throwing an error message and halt. 

 

What should be the approach?