Generate Factory Skill

Purpose

Auto-generate test data factories from database schemas, TypeScript interfaces, or model definitions. Factories produce realistic test data using Faker.js patterns, reducing test setup time by 60%.

Research Foundation

Pattern	Source	Reference
Factory Pattern	ThoughtBot	FactoryBot
Faker.js	Open Source	fakerjs.dev
Test Data Management	ISTQB	CT-TAS Test Automation Strategy
Synthetic Data	Tonic.ai	Faker Best Practices

When This Skill Applies

User needs to create test data factories
Setting up test infrastructure for new project
Existing tests use hard-coded data
Schema/model changes require test data updates
Need realistic but deterministic test data

Trigger Phrases

Natural Language	Action
"Generate factory for User model"	Create user factory
"Create test data factories"	Generate factories for all models
"Add faker to tests"	Integrate faker with existing tests
"Make test data realistic"	Convert hard-coded to factory
"Generate fixtures from schema"	Schema-aware factory generation

Factory Concepts

Factory vs Fixture vs Mock

Type	Purpose	When to Use
Factory	Generate dynamic test data	When you need many variations
Fixture	Static, predefined data	When exact values matter
Mock	Fake external dependencies	When isolating units

Factory Features

typescript

// Basic factory
const user = userFactory.build();
// { id: 'uuid-1', name: 'John Doe', email: 'john@example.com' }

// With overrides
const admin = userFactory.build({ role: 'admin' });
// { id: 'uuid-2', name: 'Jane Doe', email: 'jane@example.com', role: 'admin' }

// Build multiple
const users = userFactory.buildList(5);
// Array of 5 users

// With traits
const inactiveUser = userFactory.build({}, { trait: 'inactive' });
// { id: 'uuid-3', ..., status: 'inactive', deactivatedAt: Date }

// With relationships
const userWithOrders = userFactory.build({}, { with: ['orders'] });
// { id: 'uuid-4', ..., orders: [{ id: 'order-1', ... }] }

Generation Process

1. Analyze Source

Detect schema/type from:

TypeScript interfaces
Prisma schema
JSON Schema
Database migrations
OpenAPI specs

typescript

// Input: TypeScript interface
interface User {
  id: string;
  name: string;
  email: string;
  age: number;
  role: 'admin' | 'user' | 'guest';
  createdAt: Date;
  preferences?: UserPreferences;
}

2. Map Types to Faker

typescript

const TYPE_MAPPING = {
  // Primitives
  'string': 'faker.string.alphanumeric(10)',
  'number': 'faker.number.int({ min: 0, max: 100 })',
  'boolean': 'faker.datatype.boolean()',
  'Date': 'faker.date.past()',

  // Named fields (semantic mapping)
  'id': 'faker.string.uuid()',
  'name': 'faker.person.fullName()',
  'email': 'faker.internet.email()',
  'phone': 'faker.phone.number()',
  'address': 'faker.location.streetAddress()',
  'age': 'faker.number.int({ min: 18, max: 80 })',
  'createdAt': 'faker.date.past()',
  'updatedAt': 'faker.date.recent()',

  // Enums
  'role': 'faker.helpers.arrayElement(["admin", "user", "guest"])',
};

3. Generate Factory

typescript

// Generated: factories/user.factory.ts
import { faker } from '@faker-js/faker';
import type { User } from '../types';

export const userFactory = {
  /**
   * Build a single User with optional overrides
   */
  build: (overrides: Partial<User> = {}): User => ({
    id: faker.string.uuid(),
    name: faker.person.fullName(),
    email: faker.internet.email(),
    age: faker.number.int({ min: 18, max: 80 }),
    role: faker.helpers.arrayElement(['admin', 'user', 'guest']),
    createdAt: faker.date.past(),
    ...overrides,
  }),

  /**
   * Build multiple Users
   */
  buildList: (count: number, overrides: Partial<User> = {}): User[] =>
    Array.from({ length: count }, () => userFactory.build(overrides)),

  /**
   * Traits for common variations
   */
  traits: {
    admin: { role: 'admin' as const },
    inactive: {
      status: 'inactive',
      deactivatedAt: faker.date.past(),
    },
    newUser: {
      createdAt: faker.date.recent(),
    },
  },

  /**
   * Build with trait
   */
  buildWithTrait: (trait: keyof typeof userFactory.traits, overrides: Partial<User> = {}): User =>
    userFactory.build({ ...userFactory.traits[trait], ...overrides }),
};

4. Generate Related Factories

For entities with relationships:

typescript

// factories/order.factory.ts
import { faker } from '@faker-js/faker';
import { userFactory } from './user.factory';

export const orderFactory = {
  build: (overrides = {}) => ({
    id: faker.string.uuid(),
    userId: faker.string.uuid(),
    items: [],
    total: faker.number.float({ min: 10, max: 500, fractionDigits: 2 }),
    status: faker.helpers.arrayElement(['pending', 'shipped', 'delivered']),
    createdAt: faker.date.past(),
    ...overrides,
  }),

  /**
   * Build with related user
   */
  buildWithUser: (overrides = {}) => {
    const user = userFactory.build();
    return {
      ...orderFactory.build({ userId: user.id, ...overrides }),
      user,
    };
  },
};

Output Format

markdown

## Factory Generation Report

**Source**: `src/types/user.ts`
**Output**: `test/factories/user.factory.ts`

### Generated Factory

```typescript
import { faker } from '@faker-js/faker';
import type { User } from '../../src/types';

export const userFactory = {
  build: (overrides: Partial<User> = {}): User => ({
    id: faker.string.uuid(),
    name: faker.person.fullName(),
    email: faker.internet.email(),
    age: faker.number.int({ min: 18, max: 80 }),
    role: faker.helpers.arrayElement(['admin', 'user', 'guest']),
    createdAt: faker.date.past(),
    preferences: null,
    ...overrides,
  }),

  buildList: (count: number, overrides: Partial<User> = {}): User[] =>
    Array.from({ length: count }, () => userFactory.build(overrides)),

  traits: {
    admin: { role: 'admin' as const },
    inactive: { status: 'inactive' },
  },
};

Field Mappings

Field	Type	Faker Method
id	string	`faker.string.uuid()`
name	string	`faker.person.fullName()`
email	string	`faker.internet.email()`
age	number	`faker.number.int({ min: 18, max: 80 })`
role	enum	`faker.helpers.arrayElement([...])`
createdAt	Date	`faker.date.past()`

Usage Examples

typescript

// Basic usage
const user = userFactory.build();

// With override
const admin = userFactory.build({ role: 'admin' });

// Multiple users
const users = userFactory.buildList(10);

// With trait
const inactive = userFactory.build(userFactory.traits.inactive);

Dependencies Added

json

{
  "devDependencies": {
    "@faker-js/faker": "^8.0.0"
  }
}


## Deterministic Mode

For tests requiring reproducible data:

```typescript
// Set seed for reproducible data
import { faker } from '@faker-js/faker';

beforeEach(() => {
  faker.seed(12345);  // Same data every run
});

// Or per-factory
export const userFactory = {
  buildDeterministic: (seed: number, overrides = {}) => {
    faker.seed(seed);
    return userFactory.build(overrides);
  },
};

Batch Generation

Generate factories for all models:

bash

# From Prisma schema
npx generate-factory --source prisma/schema.prisma --output test/factories/

# From TypeScript types
npx generate-factory --source src/types/ --output test/factories/

Integration Points

Works with tdd-enforce for test data requirements
Used by Test Engineer for test creation
Feeds into integration test setup
Compatible with database seeders

Script Reference

factory_generator.py

Generate factory from schema:

bash

python scripts/factory_generator.py --source src/types/user.ts

batch_generate.py

Generate all factories:

bash

python scripts/batch_generate.py --source src/types/ --output test/factories/

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SKILL.md

Generate Factory Skill

Purpose

Research Foundation

When This Skill Applies

Trigger Phrases

Factory Concepts

Factory vs Fixture vs Mock

Factory Features

Generation Process

1. Analyze Source

2. Map Types to Faker

3. Generate Factory

4. Generate Related Factories

Output Format

Field Mappings

Usage Examples

Dependencies Added

Batch Generation

Integration Points

Script Reference

factory_generator.py

batch_generate.py