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apps/backend/src/common/security/validation.pipe.ts

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+import { ValidationPipe } from '@nestjs/common';
+import type { ValidationError } from '@nestjs/common';
+import { ErrorCode } from '../errors/error-codes';
+import { ApiError, type ValidationErrorDetail } from '../errors/api-error';
+
+function flattenValidation(
+  errors: ValidationError[],
+  parent = '',
+): ValidationErrorDetail[] {
+  return errors.flatMap((error) => {
+    const field = parent ? `${parent}.${error.property}` : error.property;
+    const own = error.constraints
+      ? [
+          {
+            field,
+            messages: Object.values(error.constraints).map(
+              () => 'Ungueltiger Wert.',
+            ),
+          },
+        ]
+      : [];
+    return [...own, ...flattenValidation(error.children ?? [], field)];
+  });
+}
+
+export function createValidationPipe(): ValidationPipe {
+  return new ValidationPipe({
+    whitelist: true,
+    forbidNonWhitelisted: true,
+    transform: true,
+    transformOptions: { enableImplicitConversion: false },
+    exceptionFactory: (errors) => {
+      const validation = flattenValidation(errors);
+      return new ApiError(
+        ErrorCode.ValidationFailed,
+        'Bitte pruefen Sie die markierten Felder.',
+        400,
+        validation,
+      );
+    },
+  });
+}