# ADR-003: Standardized Input Validation using Middleware

**Date**: 2025-12-12

**Status**: Accepted

**Implemented**: 2026-01-07

## Context

Our Express route handlers currently perform manual validation of request parameters, queries, and bodies. This involves repetitive boilerplate code using `parseInt`, `isNaN`, and type checks like `Array.isArray`. This approach has several disadvantages:

1. **Code Duplication**: The same validation logic (e.g., checking for a valid integer ID) is repeated across many different routes.
2. **Cluttered Business Logic**: Route handlers are cluttered with validation code, obscuring their primary business logic.
3. **Inconsistent Error Messages**: Manual validation can lead to inconsistent error messages for similar validation failures across the API.
4. **Error-Prone**: It is easy to forget a validation check, leading to unexpected data types being passed to service and repository layers, which could cause runtime errors.

## Decision

We will adopt a schema-based approach for input validation using the `zod` library and a custom Express middleware.

1. **Adopt `zod` for Schema Definition**: We will use `zod` to define clear, type-safe schemas for the `params`, `query`, and `body` of each API request. `zod` provides powerful and declarative validation rules and automatically infers TypeScript types.

2. **Create a Reusable Validation Middleware**: A generic `validateRequest(schema)` middleware will be created. This middleware will take a `zod` schema, parse the incoming request against it, and handle success and error cases.
   - On successful validation, the parsed and typed data will be attached to the `req` object (e.g., `req.body` will be replaced with the parsed body), and `next()` will be called.
   - On validation failure, the middleware will call `next()` with a custom `ValidationError` containing a structured list of issues, which `ADR-001`'s `errorHandler` can then format into a user-friendly `400 Bad Request` response.

3. **Refactor Routes**: All route handlers will be refactored to use this new middleware, removing all manual validation logic.

4. **(New) Resilient Type Inference**: To achieve full type safety, we will use **inline type assertions** with `z.infer<typeof schema>`. This ensures the code inside the handler is fully typed and benefits from IntelliSense without creating complex utility types that conflict with Express's `RequestHandler` signature.

### Example Usage (Refined Pattern)

```typescript
// In flyer.routes.ts

import { z } from 'zod';
import { validateRequest } from '../middleware/validation';

// 1. Define the schema
const getFlyerSchema = z.object({
  params: z.object({
    id: z.string().pipe(z.coerce.number().int().positive()),
  }),
});

// 2. Infer the type from the schema for local use
type GetFlyerRequest = z.infer<typeof getFlyerSchema>;

// 3. Apply the middleware and use an inline cast for the request
router.get('/:id', validateRequest(getFlyerSchema), async (req, res, next) => {
  // Cast 'req' to the inferred type.
  // This provides full type safety for params, query, and body.
  const { params } = req as unknown as GetFlyerRequest;

  try {
    const flyer = await db.flyerRepo.getFlyerById(params.id); // params.id is 'number'
    res.json(flyer);
  } catch (error) {
    next(error);
  }
});
```

## Consequences

### Positive

**Reduced Complexity**: Avoids maintaining a complex ValidatedRequestHandler utility type that could conflict with Express or TypeScript upgrades.
**Explicit Contracts**: By defining the Request type right next to the route, the contract for that endpoint is immediately visible.
**Improved Readability**: Route handlers become much cleaner and focus exclusively on their core business logic.
**Type Safety**: `zod` schemas provide strong compile-time and runtime type safety, reducing bugs.
**Consistent and Detailed Errors**: The `errorHandler` can be configured to provide consistent, detailed validation error messages for all routes (e.g., "Query parameter 'limit' must be a positive integer").
**Robustness**: Prevents invalid data from ever reaching the service or database layers.

### Negative

**Minor Verbosity**: Requires one extra line (type ... = z.infer<...>) and a controlled cast (as unknown as ...) within the handler function.
**New Dependency**: Introduces `zod` as a new project dependency.
**Learning Curve**: Developers need to learn the `zod` schema definition syntax.
**Refactoring Effort**: Requires a one-time effort to create schemas and refactor all existing routes to use the `validateRequest` middleware.

## Implementation Details

### The `validateRequest` Middleware

Located in `src/middleware/validation.middleware.ts`:

```typescript
export const validateRequest =
  (schema: ZodObject<z.ZodRawShape>) => async (req: Request, res: Response, next: NextFunction) => {
    try {
      const { params, query, body } = await schema.parseAsync({
        params: req.params,
        query: req.query,
        body: req.body,
      });

      // Merge parsed data back into request
      Object.keys(req.params).forEach((key) => delete req.params[key]);
      Object.assign(req.params, params);
      Object.keys(req.query).forEach((key) => delete req.query[key]);
      Object.assign(req.query, query);
      req.body = body;

      return next();
    } catch (error) {
      if (error instanceof ZodError) {
        const validationIssues = error.issues.map((issue) => ({
          ...issue,
          path: issue.path.map((p) => String(p)),
        }));
        return next(new ValidationError(validationIssues));
      }
      return next(error);
    }
  };
```

### Common Zod Patterns

```typescript
import { z } from 'zod';
import { requiredString } from '../utils/zodUtils';

// String that coerces to positive integer (for ID params)
const idParam = z.string().pipe(z.coerce.number().int().positive());

// Pagination query params with defaults
const paginationQuery = z.object({
  limit: z.coerce.number().int().positive().max(100).default(20),
  offset: z.coerce.number().int().nonnegative().default(0),
});

// Email with sanitization
const emailSchema = z.string().trim().toLowerCase().email('A valid email is required.');

// Password with strength validation
const passwordSchema = z
  .string()
  .trim()
  .min(8, 'Password must be at least 8 characters long.')
  .superRefine((password, ctx) => {
    const strength = validatePasswordStrength(password);
    if (!strength.isValid) ctx.addIssue({ code: 'custom', message: strength.feedback });
  });

// Optional string that converts empty string to undefined
const optionalString = z.preprocess(
  (val) => (val === '' ? undefined : val),
  z.string().trim().optional(),
);
```

### Routes Using `validateRequest`

All API routes use the validation middleware:

| Router                   | Schemas Defined | Validated Endpoints                                                              |
| ------------------------ | --------------- | -------------------------------------------------------------------------------- |
| `auth.routes.ts`         | 5               | `/register`, `/login`, `/forgot-password`, `/reset-password`, `/change-password` |
| `user.routes.ts`         | 4               | `/profile`, `/address`, `/preferences`, `/notifications`                         |
| `flyer.routes.ts`        | 6               | `GET /:id`, `GET /`, `GET /:id/items`, `DELETE /:id`                             |
| `budget.routes.ts`       | 5               | `/`, `/:id`, `/batch`, `/categories`                                             |
| `recipe.routes.ts`       | 4               | `GET /`, `GET /:id`, `POST /`, `PATCH /:id`                                      |
| `admin.routes.ts`        | 8               | Various admin endpoints                                                          |
| `ai.routes.ts`           | 3               | `/upload-and-process`, `/analyze`, `/jobs/:jobId/status`                         |
| `gamification.routes.ts` | 3               | `/achievements`, `/leaderboard`, `/points`                                       |

### Validation Error Response Format

When validation fails, the `errorHandler` returns:

```json
{
  "message": "The request data is invalid.",
  "errors": [
    {
      "path": ["body", "email"],
      "message": "A valid email is required."
    },
    {
      "path": ["body", "password"],
      "message": "Password must be at least 8 characters long."
    }
  ]
}
```

HTTP Status: `400 Bad Request`

### Zod Utility Functions

Located in `src/utils/zodUtils.ts`:

```typescript
// String that rejects empty strings
export const requiredString = (message?: string) =>
  z.string().min(1, message || 'This field is required.');

// Number from string with validation
export const numericString = z.string().pipe(z.coerce.number());

// Boolean from string ('true'/'false')
export const booleanString = z.enum(['true', 'false']).transform((v) => v === 'true');
```

## Key Files

- `src/middleware/validation.middleware.ts` - The `validateRequest` middleware
- `src/services/db/errors.db.ts` - `ValidationError` class definition
- `src/middleware/errorHandler.ts` - Error formatting for validation errors
- `src/utils/zodUtils.ts` - Reusable Zod schema utilities

## Related ADRs

- [ADR-001](./0001-standardized-error-handling.md) - Error handling for validation errors
- [ADR-032](./0032-rate-limiting-strategy.md) - Rate limiting applied alongside validation
