---

name: data-validation description: Provides comprehensive guidance for input validation, data serialization, and ID management in backend APIs. This skill should be used when designing validation schemas, transforming request/response data, mapping database IDs to external identifiers, and ensuring type safety across API boundaries.

Data Validation Skill

Provides comprehensive guidance for input validation, response serialization, and ID management.

Core Concepts

API Contract vs Internal Implementation:

Copy┌─────────────────────────────────────────┐
│          Client (External)              │
│    snake_case IDs, user-friendly        │
└──────────────┬──────────────────────────┘
               │
         HTTP API Boundary
               │
┌──────────────▼──────────────────────────┐
│    Your Application (Internal)          │
│    camelCase IDs, database structure    │
└─────────────────────────────────────────┘

Responsibilities at each layer:

ID Management Pattern

External API Contract

CopyURL:
  GET /admin/users/:id     // id = uid (user_abc123)

Response:
{
  "id": "user_abc123",                  // UID mapped as id
  "email": "user@example.com",
  "name": "John Doe",
  // NO "uid" field
  // NO database "id" field (bigint primary key)
}

Internal Implementation

CopyDatabase:
{
  id: 12345,                // bigint primary key (NEVER exposed)
  uid: "user_abc123",       // Branded UID
  email: "user@example.com",
  name: "John Doe",
}

Services and Repositories:
- Use uid: "user_abc123"
- Never expose id: 12345
- Query by uid: WHERE uid = 'user_abc123'

UID Format

CopyPattern: {PREFIX}_{RANDOM_ID}

Examples:
- user_abc123
- show_xyz789
- client_def456
- studio_ghi012

Key Rules:
- ✅ Prefix has no trailing underscore
- ✅ Use cryptographically secure random
- ✅ Make globally unique
- ❌ Never expose database ID pattern
- ❌ Never expose prefix pattern in error messages

Input Validation Pattern

Validate at API boundary, transform format:

CopyClient Request (snake_case):
{
  "email": "user@example.com",
  "user_id": "user_123",
  "is_banned": false
}
    ↓
Validation Layer:
- Check required fields
- Check format (email, length, etc.)
- Check references exist (user_id)
    ↓
Transform Layer (snake_case → camelCase):
{
  email: "user@example.com",
  userId: "user_123",
  isBanned: false
}
    ↓
Service Layer (processes camelCase)

Validation Schema Example:

CopyInput schema:
- email: string, email format, required
- name: string, min 1 char, max 255 chars
- is_banned: boolean, optional
- user_id: string, matches UID format

Validation rules:
- Transform snake_case → camelCase
- Check format of IDs (startsWith prefix)
- Check required fields
- Check string lengths

Response Serialization Pattern

Transform internal format to API format:

CopyService returns (camelCase, internal):
{
  id: 12345n,                 // database ID (never in response!)
  uid: "user_abc123",         // UID (maps to "id")
  email: "user@example.com",
  isBanned: false,
  createdAt: Date,
  updatedAt: Date,
}
    ↓
Serialization Layer:
- Map uid → id
- Hide database id field
- Transform camelCase → snake_case
- Transform dates to ISO format
    ↓
Client receives (snake_case, friendly):
{
  "id": "user_abc123",        // UID as id
  "email": "user@example.com",
  "is_banned": false,
  "created_at": "2025-01-14T10:00:00Z",
  "updated_at": "2025-01-14T10:00:00Z"
  // NO "uid" field
  // NO database "id" field
}

Serialization Schema Example:

CopyOutput schema (from service):
- uid: string
- email: string
- isBanned: boolean
- createdAt: Date
- updatedAt: Date

Transform to DTO:
- uid → id
- isBanned → is_banned
- createdAt → created_at
- updatedAt → updated_at

Nested Validation

Validate related entities by UID:

CopyInput (user creating a show):
{
  "name": "Studio A Show",
  "client_id": "client_123",      // Client UID
  "studio_room_id": "room_456",   // StudioRoom UID
  "show_type_id": "type_bau",     // ShowType UID
}

Validation:
1. Check string format (looks like UID)
2. Service verifies entity exists
3. Service queries by UID
4. If not found, throw not-found error

Key Rules:

• ✅ Validate UID format (starts with prefix)
• ✅ Service verifies entity exists (query)
• ✅ Return not-found error if missing
• ❌ Never assume IDs exist without checking
• ❌ Never expose missing ID in error details

Type Mapping

Database → Service → API:

CopyDatabase     | Service       | API Response
─────────────┼───────────────┼──────────────
bigint       | bigint        | string (UID)
string (uid) | string (uid)  | string (id)
boolean      | boolean       | boolean
timestamp    | Date          | ISO string

Transformation Examples:

CopyDatabase integer → API string (UID):
  DB: { id: 12345, uid: "user_abc123" }
  Service: { uid: "user_abc123" }
  API: { "id": "user_abc123" }

Database TIMESTAMP → API ISO string:
  DB: { created_at: 2025-01-14 10:00:00 }
  Service: { createdAt: Date(2025-01-14 10:00:00) }
  API: { "created_at": "2025-01-14T10:00:00Z" }

Database boolean → API boolean:
  DB: { is_banned: true }
  Service: { isBanned: true }
  API: { "is_banned": true }

Pagination Validation

Validate pagination parameters:

CopyInput:
{
  "page": "1",      // String from query param
  "limit": "10"
}

Validation:
- Convert to number
- Check >= 1
- Check <= max (e.g., 100)
- Provide defaults (page: 1, limit: 10)

Output:
{
  page: 1,
  limit: 10
}

Error Messages

Security-conscious error messages:

Copy✅ GOOD: Context-specific, doesn't expose internals
{
  "statusCode": 404,
  "message": "User not found",
  "error": "NotFound"
}

✅ GOOD: Validation error with field info
{
  "statusCode": 400,
  "message": "Validation failed",
  "error": "BadRequest",
  "details": [
    { "field": "email", "message": "Invalid email format" }
  ]
}

❌ BAD: Exposes internal ID format
{
  "statusCode": 404,
  "message": "User uid_123 not found"  // Reveals UID pattern
}

❌ BAD: Exposes database structure
{
  "statusCode": 404,
  "message": "No row with id 12345 found"  // Reveals database ID
}

Best Practices Checklist

• Validate all input at controller boundary
• Use schema validation (Zod, Joi, Pydantic)
• Transform snake_case → camelCase on input
• Map uid → id in API responses
• Hide database primary keys completely
• Transform camelCase → snake_case on output
• Check UID format (matches prefix pattern)
• Validate referenced entities exist
• Return not-found error if entity missing
• Support pagination with validation
• Convert timestamps to ISO format
• Error messages don't expose internal structure
• Error messages are actionable for clients
• Serialization is consistent across endpoints
• No sensitive data in responses

Related Skills

• backend-controller-pattern/SKILL.md - Validation at HTTP boundary
• service-pattern/SKILL.md - Business logic validation
• repository-pattern/SKILL.md - Data access layer