fix: reject a negative quantity on IN/OUT stock movements

addMovement() negates quantity internally for OUT (delta = -quantity)
and applies it directly for IN. Nothing stopped a negative quantity
from being submitted, so e.g. { type: 'OUT', quantity: -5 } would
compute delta = -(-5) = +5 - silently ADDING stock while being
recorded in the audit trail as a removal. The inverse held for IN.
ADJUSTMENT is the one type that must stay allowed to be negative,
since that's the only way to correct a stock count downward, so this
is a service-level guard (BadRequestException) rather than a DTO
validator - class-validator's @ValidateIf skips every validator on a
property when its condition is false, including @IsNumber, so it
can't express "always a number, but only sometimes positive" cleanly;
verified this empirically before choosing the service-level approach.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

ApiCore-Backend/src/storage/dto/create-stock-movement.dto.ts

@@ -7,7 +7,11 @@ export class CreateStockMovementDto {
   @IsEnum(StockMovementType)
   type!: StockMovementType;
 
-  @ApiProperty()
+  @ApiProperty({
+    description:
+      'Für IN/OUT immer positiv angeben - die Richtung ergibt sich aus "type". ' +
+      'Nur ADJUSTMENT erlaubt einen negativen Wert, um den Bestand direkt zu korrigieren.',
+  })
   @IsNumber()
   quantity!: number;
 

ApiCore-Backend/src/storage/storage.service.spec.ts

@@ -1,4 +1,4 @@
-import { NotFoundException } from '@nestjs/common';
+import { BadRequestException, NotFoundException } from '@nestjs/common';
 import { StorageService } from './storage.service';
 import { PrismaService } from '../prisma/prisma.service';
 import { AuditLogService } from '../audit/audit-log.service';
@@ -100,4 +100,36 @@ describe('StorageService', () => {
 
     expect(tx.stockItem.update).not.toHaveBeenCalled();
   });
+
+  it('addMovement rejects a negative quantity for OUT instead of silently adding stock', async () => {
+    prisma.stockItem.findUnique.mockResolvedValue(stockItem);
+
+    await expect(
+      service.addMovement('item-1', { type: 'OUT', quantity: -3 }, 'user-1'),
+    ).rejects.toBeInstanceOf(BadRequestException);
+    expect(tx.stockItem.update).not.toHaveBeenCalled();
+  });
+
+  it('addMovement rejects a negative quantity for IN', async () => {
+    prisma.stockItem.findUnique.mockResolvedValue(stockItem);
+
+    await expect(
+      service.addMovement('item-1', { type: 'IN', quantity: -3 }, 'user-1'),
+    ).rejects.toBeInstanceOf(BadRequestException);
+  });
+
+  it('addMovement allows a negative quantity for ADJUSTMENT', async () => {
+    prisma.stockItem.findUnique.mockResolvedValue(stockItem);
+
+    await service.addMovement(
+      'item-1',
+      { type: 'ADJUSTMENT', quantity: -3 },
+      'user-1',
+    );
+
+    const [[updateArg]] = tx.stockItem.update.mock.calls as [
+      [{ data: { quantity: { increment: number } } }],
+    ];
+    expect(updateArg.data.quantity.increment).toBe(-3);
+  });
 });

ApiCore-Backend/src/storage/storage.service.ts

@@ -1,4 +1,8 @@
-import { Injectable, NotFoundException } from '@nestjs/common';
+import {
+  BadRequestException,
+  Injectable,
+  NotFoundException,
+} from '@nestjs/common';
 import { PrismaService } from '../prisma/prisma.service';
 import { AuditLogService } from '../audit/audit-log.service';
 import { WebhooksService } from '../webhooks/webhooks.service';
@@ -123,6 +127,16 @@ export class StorageService {
     actingUserId: string,
   ) {
     await this.findStockItemById(id);
+    // IN/OUT direction comes entirely from `type`, which then negates
+    // quantity for OUT below - a negative quantity submitted for OUT would
+    // therefore silently ADD stock while being logged as a removal,
+    // inverting the audit trail. Only ADJUSTMENT may be negative, since
+    // that is the only way to correct a stock count downward.
+    if (dto.type !== 'ADJUSTMENT' && dto.quantity <= 0) {
+      throw new BadRequestException(
+        'quantity must be positive for IN/OUT movements - the direction is determined by "type"',
+      );
+    }
     const delta =
       dto.type === 'IN' || dto.type === 'ADJUSTMENT'
         ? dto.quantity