Quick fix

app/Exceptions/ConflictException.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Exceptions;
+
+use Exception;
+
+/**
+ * Class ConflictException.
+ *
+ * @codeCoverageIgnore
+ */
+class ConflictException extends Exception {}

app/Exceptions/Handler.php

@@ -106,5 +106,14 @@ class Handler extends ExceptionHandler
                 ], 401);
             }
         });
+
+        $this->renderable(function (ConflictException $exception, $request) {
+            return response()->json([
+                'message' => 'conflict',
+                'reason'  => [$exception->getMessage()],
+            ], 409);
+        });
+
+        
     }
 }

app/Services/GroupService.php

@@ -2,11 +2,13 @@
 
 namespace App\Services;
 
+use App\Exceptions\ConflictException;
 use App\Models\Group;
 use App\Models\TwoFAccount;
 use App\Models\User;
 use Illuminate\Auth\Access\AuthorizationException;
 use Illuminate\Database\Eloquent\Collection;
+use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Log;
 
 class GroupService
@@ -19,6 +21,7 @@ class GroupService
      * @param  mixed  $targetGroup  The group the accounts should be assigned to
      *
      * @throws \Illuminate\Auth\Access\AuthorizationException
+     * @throws App\Exceptions\ConflictException
      */
     public static function assign($ids, User $user, mixed $targetGroup = null) : void
     {
@@ -54,16 +57,27 @@ class GroupService
 
         if ($group) {
             $ids = is_array($ids) ? $ids : [$ids];
-            $twofaccounts = TwoFAccount::find($ids);
+
+            DB::transaction(function () use ($group, $ids, $user) {
+                // Check if group still exists within transaction
+                $group        = Group::lockForUpdate()->find($group->id);
+                $twofaccounts = TwoFAccount::sharedLock()->find($ids);
 
                 if ($user->cannot('updateEach', [(new TwoFAccount), $twofaccounts])) {
                     throw new AuthorizationException;
                 }
     
+                if (! $group) {
+                    throw new ConflictException(__('errors.resource_not_found'));
+                }
+
+                // Proceed with assignment
                 $group->twofaccounts()->saveMany($twofaccounts);
                 $group->loadCount('twofaccounts');
     
                 Log::info(sprintf('Twofaccounts #%s assigned to group %s (ID #%s)', implode(',', $ids), var_export($group->name, true), $group->id));
+            });
+
         } else {
             Log::info('Cannot find a group to assign the TwoFAccounts to');
         }

resources/js/components/DestinationGroupSelector.vue

@@ -21,7 +21,7 @@
         if (destinationGroupId.value === 0) {
             await twofaccountService.withdraw(props.selectedAccountsIds)
         }
-        else await groupService.assign(props.selectedAccountsIds, destinationGroupId.value)
+        else groupService.assign(props.selectedAccountsIds, destinationGroupId.value, { returnError: true })
 
         emit('accounts-moved')
     }