# Backend Development Rules for AI/LLM

This document provides guidelines and rules that AI/LLM assistants should follow when writing or modifying backend code for the Crafting Shop application.

## Architecture Principles

### Application Factory Pattern
- **ALWAYS** use the application factory pattern via `create_app()` in `backend/app/__init__.py`
- **NEVER** initialize Flask extensions globally - create them at module level, initialize in `create_app()`
- **NEVER** create multiple SQLAlchemy instances - use the `db` extension from `app/__init__.py`

```python
# ✅ CORRECT
from app import db

class User(db.Model):
    pass

# ❌ WRONG
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
```

### Import Patterns
- Import `db` and other extensions from `app` module, not `flask_sqlalchemy` directly
- Import models from the `app.models` package (e.g., `from app.models import User`)
- All routes should import `db` from `app`

```python
# ✅ CORRECT
from app import db
from app.models import User, Product, Order

# ❌ WRONG
from flask_sqlalchemy import SQLAlchemy
from app.models.user import User  # Don't import from individual files
```

## Code Style

### Formatting
- Use double quotes for strings and dictionary keys
- Follow PEP 8 style guide
- Use meaningful variable and function names
- Maximum line length: 100 characters

### Type Hints
- Add type hints to function signatures where appropriate
- Use Python 3.11+ type hinting features

## Model Rules

### Database Models
- **ALL** models must import `db` from `app`: `from app import db`
- **ALL** models must inherit from `db.Model`
- **NEVER** create your own `db = SQLAlchemy()` instance
- Use `__tablename__` explicitly
- Use `to_dict()` method for serialization
- Use `__repr__()` for debugging

```python
# ✅ CORRECT
from app import db
from datetime import datetime

class Product(db.Model):
    __tablename__ = "products"
    
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(200), nullable=False, index=True)
    created_at = db.Column(db.DateTime, default=datetime.utcnow)
    
    def to_dict(self):
        return {"id": self.id, "name": self.name}
```

### Relationships
- Use `back_populates` for bidirectional relationships
- Use `lazy="dynamic"` for one-to-many collections when appropriate
- Define foreign keys explicitly with `db.ForeignKey()`

## Route/API Rules

### Blueprint Usage
- **ALL** routes must be defined in blueprints
- Register blueprints in `create_app()` in `backend/app/__init__.py`
- Group related routes by functionality

### Request Handling
- Use `request.get_json()` for JSON payloads
- Always validate input data
- Return proper HTTP status codes
- Return JSON responses with `jsonify()`

```python
# ✅ CORRECT
from flask import Blueprint, request, jsonify
from app import db

api_bp = Blueprint("api", __name__)

@api_bp.route("/products", methods=["POST"])
@jwt_required()
def create_product():
    data = request.get_json()
    
    if not data or not data.get("name"):
        return jsonify({"error": "Name is required"}), 400
    
    product = Product(name=data["name"])
    db.session.add(product)
    db.session.commit()
    
    return jsonify(product.to_dict()), 201
```

### Database Operations
- **NEVER** access `db` through `current_app.extensions['sqlalchemy'].db`
- **ALWAYS** import `db` from `app`
- Use `db.session.add()` and `db.session.commit()` for transactions
- Use `db.session.flush()` when you need the ID before commit
- **ALWAYS** use `db.session.get(Model, id)` instead of `Model.query.get(id)` (SQLAlchemy 2.0)
- Use `Model.query.get_or_404(id)` for 404 handling when appropriate
```python
# ✅ CORRECT - SQLAlchemy 2.0 syntax
from app import db
from app.models import User

user = db.session.get(User, user_id)

# ❌ WRONG - Legacy syntax (deprecated)
user = User.query.get(user_id)
```

### Error Handling
- Handle common errors (404, 400, 401, 403, 500)
- Return JSON error responses with consistent format
- Use Flask error handlers where appropriate

```python
# ✅ CORRECT
@app.errorhandler(404)
def not_found(error):
    return jsonify({"error": "Not found"}), 404
```

### Authentication
- Use `@jwt_required()` decorator for protected routes
- Use `get_jwt_identity()` to get current user ID
- Verify user permissions in routes

```python
# ✅ CORRECT
from flask_jwt_extended import jwt_required, get_jwt_identity

@api_bp.route("/orders", methods=["POST"])
@jwt_required()
def create_order():
    user_id = get_jwt_identity()
    # ... rest of code
```

## Configuration

### Environment Variables
- **ALL** configuration must use environment variables via `os.environ.get()`
- Provide sensible defaults in `app/config.py`
- Use `.env.example` for documentation

### CORS
- Configure CORS in `create_app()` using the `cors` extension
- Use `CORS_ORIGINS` environment variable

```python
# ✅ CORRECT
cors.init_app(app, resources={r"/api/*": {"origins": app.config.get('CORS_ORIGINS', '*')}})
```

## Service Layer Rules

### Business Logic
- Place complex business logic in `backend/app/services/`
- Services should be stateless functions or classes
- Services should import `db` from `app`

```python
# ✅ CORRECT
from app import db
from app.models import Product

def create_product_service(data):
    product = Product(**data)
    db.session.add(product)
    db.session.commit()
    return product
```

## Testing Rules

### Test Structure
- Use pytest framework
- Place tests in `backend/tests/`
- Use fixtures for common setup
- Organize tests by functionality: `test_models.py`, `test_routes.py`, `test_schemas.py`

### Test Naming Conventions
- Test files must start with `test_`: `test_products.py`, `test_users.py`
- Test classes must start with `Test`: `TestProductModel`, `TestAuthRoutes`
- Test functions must start with `test_`: `test_create_product`, `test_login_success`
- Use descriptive names: `test_create_product_with_valid_data` (not `test_product`)

### Writing Tests

#### Basic Test Structure

```python
# ✅ CORRECT
import pytest
from app import db
from app.models import Product

class TestProductModel:
    """Test Product model"""
    
    @pytest.mark.unit
    def test_product_creation(self, db_session):
        """Test creating a product with valid data"""
        product = Product(
            name='Test Product',
            price=99.99
        )
        db_session.add(product)
        db_session.commit()
        
        assert product.id is not None
        assert product.name == 'Test Product'
```

#### Testing API Routes

```python
# ✅ CORRECT
def test_create_product(client, admin_headers):
    """Test creating a product as admin"""
    response = client.post('/api/products',
                         headers=admin_headers,
                         json={
                             'name': 'New Product',
                             'price': 29.99
                         })
    
    assert response.status_code == 201
    data = response.get_json()
    assert data['name'] == 'New Product'
    assert 'password' not in data
```

#### Using Fixtures

```python
# ✅ CORRECT - Use available fixtures
def test_get_products(client, products):
    """Test getting all products"""
    response = client.get('/api/products')
    
    assert response.status_code == 200
    data = response.get_json()
    assert len(data) == 5

# ❌ WRONG - Don't create fixtures manually in tests
def test_get_products_wrong(client, db_session):
    products = []
    for _ in range(5):
        p = Product(name='Test', price=10)
        db_session.add(p)
        products.append(p)
    db_session.commit()
    # ... use fixtures instead!
```

#### Testing Both Success and Failure Cases

```python
# ✅ CORRECT - Test both scenarios
def test_create_product_success(client, admin_headers):
    """Test creating product successfully"""
    response = client.post('/api/products',
                         headers=admin_headers,
                         json={'name': 'Test', 'price': 10})
    assert response.status_code == 201

def test_create_product_unauthorized(client):
    """Test creating product without authentication"""
    response = client.post('/api/products',
                         json={'name': 'Test', 'price': 10})
    assert response.status_code == 401

def test_create_product_validation_error(client, admin_headers):
    """Test creating product with invalid data"""
    response = client.post('/api/products',
                         headers=admin_headers,
                         json={'name': 'Test', 'price': -10})
    assert response.status_code == 400
```

#### Parameterized Tests

```python
# ✅ CORRECT - Use parameterization for similar tests
@pytest.mark.parametrize("email,password,expected_status", [
    ("user@example.com", "correct123", 200),
    ("wrong@email.com", "correct123", 401),
    ("user@example.com", "wrongpass", 401),
])
def test_login_validation(client, email, password, expected_status):
    """Test login with various invalid inputs"""
    response = client.post('/api/auth/login', json={
        'email': email,
        'password': password
    })
    assert response.status_code == expected_status
```

### Test Markers

Use appropriate markers for categorizing tests:

```python
# ✅ CORRECT
@pytest.mark.unit
def test_user_creation(self, db_session):
    """Unit test - no HTTP, no external services"""
    pass

@pytest.mark.integration
def test_user_workflow(self, client):
    """Integration test - full request/response cycle"""
    pass

@pytest.mark.auth
def test_login(self, client):
    """Authentication-related test"""
    pass

@pytest.mark.product
def test_get_products(self, client):
    """Product-related test"""
    pass
```

### Database in Tests
- Use in-memory SQLite for tests
- Clean up database between tests
- Use `pytest.fixture` for database setup
- **NEVER** use production database in tests
- **NEVER** share state between tests

```python
# ✅ CORRECT - Use db_session fixture
def test_something(db_session):
    user = User(email='test@example.com')
    db_session.add(user)
    db_session.commit()
    
# ❌ WRONG - Don't access db directly
def test_something_wrong():
    from app import db
    user = User(email='test@example.com')
    db.session.add(user)
    db.session.commit()
```

### Available Fixtures

Use these fixtures from `tests/conftest.py`:

- **`app`**: Flask application instance with test configuration
- **`client`**: Test client for making HTTP requests
- **`runner`**: CLI runner for Flask commands
- **`db_session`**: Database session for database operations
- **`admin_user`**: Pre-created admin user
- **`regular_user`**: Pre-created regular user
- **`inactive_user`**: Pre-created inactive user
- **`product`**: Single product
- **`products`**: Multiple products (5 items)
- **`auth_headers`**: JWT headers for regular user
- **`admin_headers`**: JWT headers for admin user
- **`order`**: Pre-created order with items

### Creating Custom Fixtures

```python
# In tests/conftest.py or test file
@pytest.fixture
def custom_resource(db_session):
    """Create a custom test resource"""
    resource = CustomModel(
        name='Test Resource',
        value=100
    )
    db_session.add(resource)
    db_session.commit()
    return resource

# Use in tests
def test_custom_fixture(custom_resource):
    assert custom_resource.name == 'Test Resource'
```

### Running Tests

```bash
# Run all tests
make test-backend

# Run with coverage
make test-backend-cov

# Run with verbose output
make test-backend-verbose

# Run specific test file
make test-backend-specific TEST=test_models.py

# Run by marker
make test-backend-marker MARKER=auth

# Run only failed tests
make test-backend-failed

# Run in parallel (faster)
make test-backend-parallel
```

### Test Coverage Requirements

- **Minimum 80%** code coverage required
- **Critical paths** (auth, payments, data modification) must have >90% coverage
- All new features must include tests

```python
# ✅ CORRECT - Comprehensive test coverage
def test_product_crud(self, client, admin_headers):
    """Test complete CRUD operations"""
    # Create
    response = client.post('/api/products', 
                         headers=admin_headers,
                         json={'name': 'Test', 'price': 10})
    assert response.status_code == 201
    product_id = response.get_json()['id']
    
    # Read
    response = client.get(f'/api/products/{product_id}')
    assert response.status_code == 200
    
    # Update
    response = client.put(f'/api/products/{product_id}',
                        headers=admin_headers,
                        json={'name': 'Updated', 'price': 20})
    assert response.status_code == 200
    
    # Delete
    response = client.delete(f'/api/products/{product_id}',
                         headers=admin_headers)
    assert response.status_code == 200
```

### Mocking External Services

```python
# ✅ CORRECT - Mock external dependencies
def test_external_api_call(client, mocker):
    """Test endpoint that calls external API"""
    mock_response = {'data': 'mocked data'}
    
    # Mock requests.get
    mock_get = mocker.patch('requests.get')
    mock_get.return_value.json.return_value = mock_response
    mock_get.return_value.status_code = 200
    
    response = client.get('/api/external-data')
    assert response.status_code == 200
    assert response.get_json() == mock_response
    mock_get.assert_called_once()
```

### Test DOs and DON'Ts

✅ **DO:**
- Use descriptive test names
- Test both success and failure cases
- Use fixtures for common setup
- Mock external services
- Keep tests independent
- Use markers appropriately
- Test edge cases and boundary conditions

❌ **DON'T:**
- Share state between tests
- Hardcode sensitive data (use faker)
- Use production database
- Skip error case testing
- Write tests after deployment
- Ignore slow tests in CI
- Use complex setup in test methods (use fixtures instead)

## Security Rules

### Password Handling
- **NEVER** store plain text passwords
- Use `werkzeug.security` for password hashing
- Use `set_password()` and `check_password()` methods

### SQL Injection Prevention
- **ALWAYS** use SQLAlchemy ORM, never raw SQL
- Use parameterized queries if raw SQL is absolutely necessary

### Input Validation
- Validate all user inputs
- Sanitize data before database operations
- Use Flask-WTF for form validation

## File Structure for New Features

When adding new features, follow this structure:

```
backend/app/
├── models/
│   └── feature_name.py          # Database models
├── routes/
│   └── feature_name.py           # API routes
├── services/
│   └── feature_name.py           # Business logic
└── utils/
    └── feature_name.py           # Utility functions
```

## Common Patterns

### Creating a New Model

```python
from app import db
from datetime import datetime

class NewModel(db.Model):
    __tablename__ = "new_model"
    
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(200), nullable=False, index=True)
    created_at = db.Column(db.DateTime, default=datetime.utcnow)
    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
    
    def to_dict(self):
        return {
            "id": self.id,
            "name": self.name,
            "created_at": self.created_at.isoformat() if self.created_at else None
        }
```

### Creating New Routes

```python
from flask import Blueprint, request, jsonify
from flask_jwt_extended import jwt_required, get_jwt_identity
from app import db
from app.models import NewModel

new_bp = Blueprint("new", __name__)

@new_bp.route("/resources", methods=["GET"])
def get_resources():
    resources = NewModel.query.all()
    return jsonify([r.to_dict() for r in resources]), 200

@new_bp.route("/resources", methods=["POST"])
@jwt_required()
def create_resource():
    data = request.get_json()
    resource = NewModel(**data)
    db.session.add(resource)
    db.session.commit()
    return jsonify(resource.to_dict()), 201
```

### Registering New Blueprints

In `backend/app/__init__.py`:

```python
# Import models
from app.models.new_model import NewModel

# Register blueprint
from app.routes.feature_name import new_bp
app.register_blueprint(new_bp, url_prefix="/api/new")
```

## DO NOT DO List

❌ **NEVER** create `db = SQLAlchemy()` in model files
❌ **NEVER** access `db` via `current_app.extensions['sqlalchemy'].db`
❌ **NEVER** initialize Flask extensions before `create_app()` is called
❌ **NEVER** use raw SQL queries without proper escaping
❌ **NEVER** store secrets in code (use environment variables)
❌ **NEVER** return plain Python objects (use `jsonify()`)
❌ **NEVER** use single quotes for dictionary keys
❌ **NEVER** commit transactions without proper error handling
❌ **NEVER** ignore CORS configuration
❌ **NEVER** skip input validation

## Checklist Before Committing

- [ ] All models import `db` from `app`
- [ ] All routes import `db` from `app`
- [ ] No `db = SQLAlchemy()` in model files
- [ ] All routes are in blueprints
- [ ] Proper error handling in place
- [ ] Environment variables used for configuration
- [ ] Type hints added to functions
- [ ] Tests written for new functionality
- [ ] Documentation updated
- [ ] Code follows PEP 8 style guide