django-web-patterns

name	django-web-patterns
description	Enforces Django REST Framework patterns with Pragmatic DDD: ViewSets, Serializers, Permissions, exception handling, settings, middleware. Activates when working with Django views, endpoints, or DRF serializers.

Django REST Framework Patterns (Pragmatic DDD)

These rules are NON-NEGOTIABLE. Violating any of them is a bug.

Architecture:

• NEVER put business logic in ViewSets/APIViews — ALWAYS delegate to services or model methods
• NEVER return raw Response(data) with manually constructed dicts — ALWAYS use Serializers
• NEVER use @api_view for complex endpoints — use APIView or ViewSet classes

Views:

• NEVER do manual authentication checks in views — ALWAYS use permission_classes and authentication_classes
• ALWAYS use ModelViewSet / ReadOnlyModelViewSet when providing full CRUD on a model
• ALWAYS use @action decorator for custom ViewSet actions (not extra URL patterns)
• ALWAYS declare explicit status_code on mutating @action decorators

Serializers:

• NEVER use Meta.fields = "__all__" — ALWAYS use an explicit field list
• NEVER put business logic in serializers — only validation and data transformation
• ALWAYS separate <Entity>CreateSerializer, <Entity>UpdateSerializer, <Entity>ResponseSerializer for non-trivial resources
• ALWAYS use serializers.PrimaryKeyRelatedField or nested serializer — NEVER raw ID integer fields
• ALWAYS override create()/update() in serializer when custom logic needed (not in ViewSet)

URLs:

• ALWAYS use DefaultRouter for ViewSet registration
• ALWAYS namespace URL patterns per app (app_name = "orders")
• NEVER hardcode URLs — ALWAYS use reverse() or reverse_lazy()

Permissions:

• ALWAYS use custom permission classes (not inline checks in views)
• ALWAYS combine permissions with & / | operators for composite permissions

Exception Handling:

• NEVER use DRF's default exception handler for domain errors — ALWAYS use a custom exception_handler mapping domain exceptions to API responses
• Domain exceptions hierarchy: DomainError → EntityNotFoundError, DomainValidationError, PermissionDeniedError, ConflictError (prefixed to avoid shadowing Python built-in PermissionError and Django/DRF ValidationError)

Settings & Middleware:

• ALWAYS use django-environ or pydantic-settings for env config — NEVER hardcode secrets
• ALWAYS split settings: base.py, local.py, production.py, test.py
• NEVER remove Django security middleware (SecurityMiddleware, CsrfViewMiddleware)
• ALWAYS load CORS_ALLOWED_ORIGINS from environment (via django-cors-headers)

Throttling & Filtering:

• ALWAYS configure throttle rates in settings, not per-view
• ALWAYS use django-filter with FilterSet classes — NEVER manual request.query_params parsing for filtering

Quality:

• ALWAYS run the project's typecheck and test commands after any change

These are the Django REST Framework patterns for building APIs in AppVerk projects, following Pragmatic DDD. Django Models serve as rich domain models. All patterns target Python 3.13+, Django 5.1+, Django REST Framework 3.15+.

Architecture Overview

Presentation (DRF ViewSets/APIViews, Serializers, Permissions, URLs, exception handlers)
    ↓ depends on
Application Layer (Services / Use Cases)
    ↓ depends on
Domain Layer (Django Models as rich domain models, Value Objects, Repository Protocols, Domain Exceptions)
    ↓
Infrastructure Layer (Managers, QuerySets, external integrations, Celery tasks)

Key difference from Full DDD: Django Models ARE the domain entities — there is no separate mapping layer. Business methods live directly on the Model. The infrastructure layer provides custom Managers and QuerySets that encapsulate query logic.

Directory Layout

project/
    apps/
        orders/
            models.py          # Domain models (rich domain entities)
            serializers.py     # Request/response serializers
            views.py           # ViewSets and APIViews
            urls.py            # Router registration (app_name = "orders")
            permissions.py     # Custom permission classes
            filters.py         # FilterSet classes
            services.py        # Application layer (use cases)
            exceptions.py      # Domain exception hierarchy
            tasks.py           # Celery tasks
            tests/
                test_views.py
                test_models.py
                test_services.py
    config/
        settings/
            base.py
            local.py
            production.py
            test.py
        urls.py
        wsgi.py
        asgi.py

ViewSet Structure

One ViewSet per domain resource. Custom actions use @action, not separate URL patterns.

# apps/orders/views.py
from rest_framework import status
from rest_framework.decorators import action
from rest_framework.request import Request
from rest_framework.response import Response
from rest_framework.viewsets import ModelViewSet

from apps.orders.filters import OrderFilter
from apps.orders.models import Order
from apps.orders.permissions import IsOrderOwner
from apps.orders.serializers import (
    OrderCreateSerializer,
    OrderResponseSerializer,
    OrderUpdateSerializer,
)
from apps.orders.services import OrderService


class OrderViewSet(ModelViewSet):
    permission_classes = [IsOrderOwner]
    filterset_class = OrderFilter

    def get_queryset(self):
        return Order.objects.for_user(self.request.user).select_related("product")

    def get_serializer_class(self):
        if self.action == "create":
            return OrderCreateSerializer
        if self.action in ("update", "partial_update"):
            return OrderUpdateSerializer
        return OrderResponseSerializer

    def perform_create(self, serializer: OrderCreateSerializer) -> None:
        # Delegate to service — never put logic directly here
        OrderService.create_order(
            user=self.request.user,
            validated_data=serializer.validated_data,
        )

    @action(detail=True, methods=["post"], url_path="cancel", status_code=status.HTTP_200_OK)
    def cancel(self, request: Request, pk: int | None = None) -> Response:
        order = self.get_object()
        OrderService.cancel_order(order=order, cancelled_by=request.user)
        return Response(OrderResponseSerializer(order).data)

Register with DefaultRouter in urls.py:

# apps/orders/urls.py
from django.urls import path, include
from rest_framework.routers import DefaultRouter

from apps.orders.views import OrderViewSet

app_name = "orders"

router = DefaultRouter()
router.register("", OrderViewSet, basename="order")

urlpatterns = [path("orders/", include(router.urls))]

Serializer Patterns

Separate serializers for create, update, and response. Nested serializers for related objects.

# apps/orders/serializers.py
from rest_framework import serializers

from apps.orders.models import Order
from apps.products.models import Product
from apps.products.serializers import ProductSummarySerializer


class OrderCreateSerializer(serializers.ModelSerializer):
    product = serializers.PrimaryKeyRelatedField(queryset=Product.objects.all())

    class Meta:
        model = Order
        fields = ["product", "quantity", "notes"]

    def validate_quantity(self, value: int) -> int:
        if value < 1:
            raise serializers.ValidationError("Quantity must be at least 1.")
        return value


class OrderUpdateSerializer(serializers.ModelSerializer):
    class Meta:
        model = Order
        fields = ["quantity", "notes"]


class OrderResponseSerializer(serializers.ModelSerializer):
    product = ProductSummarySerializer(read_only=True)
    status_display = serializers.CharField(source="get_status_display", read_only=True)

    class Meta:
        model = Order
        fields = [
            "id", "product", "quantity", "notes",
            "status", "status_display", "created_at", "updated_at",
        ]
        read_only_fields = fields

Conventions

• <Entity>CreateSerializer — write path for creation; validate business constraints here.
• <Entity>UpdateSerializer — write path for updates; all fields optional if supporting PATCH.
• <Entity>ResponseSerializer — read path; all fields read_only. Use nested serializers for related objects.
• Override create() / update() in the serializer (not ViewSet's perform_create) when custom save logic is needed.

Domain Exception Handling

Services raise domain exceptions. A custom DRF exception_handler maps them to HTTP responses. ViewSets never catch domain exceptions manually.

Domain Exception Hierarchy

# apps/orders/exceptions.py — no external dependencies
class DomainError(Exception):
    """Base for all domain exceptions."""


class EntityNotFoundError(DomainError):
    """Raised when a requested entity does not exist."""


class DomainValidationError(DomainError):
    """Raised when a domain business rule is violated."""


class PermissionDeniedError(DomainError):
    """Raised when the user lacks permission for the operation."""


class ConflictError(DomainError):
    """Raised when an operation conflicts with existing state (e.g. duplicate)."""

Custom DRF Exception Handler

# config/exception_handler.py
from rest_framework.exceptions import APIException
from rest_framework.views import exception_handler as drf_default_handler
from rest_framework.request import Request
from rest_framework.response import Response

from apps.orders.exceptions import (
    ConflictError,
    DomainError,
    DomainValidationError,
    EntityNotFoundError,
    PermissionDeniedError,
)

DOMAIN_EXCEPTION_MAP: dict[type[DomainError], int] = {
    EntityNotFoundError: 404,
    PermissionDeniedError: 403,
    DomainValidationError: 422,
    ConflictError: 409,
}


def custom_exception_handler(exc: Exception, context: dict) -> Response | None:
    for exc_class, status_code in DOMAIN_EXCEPTION_MAP.items():
        if isinstance(exc, exc_class):
            return Response(
                {"detail": str(exc), "error_code": type(exc).__name__},
                status=status_code,
            )
    # Fall back to DRF default for APIException, 404, etc.
    return drf_default_handler(exc, context)

Register in settings:

# config/settings/base.py
REST_FRAMEWORK = {
    "EXCEPTION_HANDLER": "config.exception_handler.custom_exception_handler",
}

Permissions

Custom permission classes encapsulate authorization logic. Combine with & / | for composite rules.

# apps/orders/permissions.py
from rest_framework.permissions import BasePermission, IsAuthenticated
from rest_framework.request import Request
from rest_framework.views import APIView

from apps.orders.models import Order


class IsOrderOwner(BasePermission):
    """Allow access only to the owner of the order."""

    def has_permission(self, request: Request, view: APIView) -> bool:
        return bool(request.user and request.user.is_authenticated)

    def has_object_permission(self, request: Request, view: APIView, obj: Order) -> bool:
        return obj.user_id == request.user.pk


# Composite: authenticated AND (owner OR staff)
class IsOwnerOrStaff(BasePermission):
    def has_object_permission(self, request: Request, view: APIView, obj: Order) -> bool:
        return obj.user_id == request.user.pk or request.user.is_staff


# Usage in ViewSet:
# permission_classes = [IsAuthenticated & IsOrderOwner]

Settings & Middleware

Split settings by environment. Load all secrets from environment variables via django-environ.

# config/settings/base.py
import environ

env = environ.Env()

SECRET_KEY = env("SECRET_KEY")
DEBUG = env.bool("DEBUG", default=False)

DATABASES = {
    "default": env.db("DATABASE_URL"),
}

INSTALLED_APPS = [
    "django.contrib.auth",
    "django.contrib.contenttypes",
    "rest_framework",
    "corsheaders",
    "django_filters",
    "apps.orders",
]

MIDDLEWARE = [
    "django.middleware.security.SecurityMiddleware",  # NEVER remove
    "corsheaders.middleware.CorsMiddleware",
    "django.middleware.common.CommonMiddleware",
    "django.middleware.csrf.CsrfViewMiddleware",  # NEVER remove
    "django.contrib.auth.middleware.AuthenticationMiddleware",
]

CORS_ALLOWED_ORIGINS = env.list("CORS_ALLOWED_ORIGINS", default=[])

REST_FRAMEWORK = {
    "DEFAULT_THROTTLE_CLASSES": [
        "rest_framework.throttling.AnonRateThrottle",
        "rest_framework.throttling.UserRateThrottle",
    ],
    "DEFAULT_THROTTLE_RATES": {
        "anon": env("THROTTLE_ANON_RATE", default="100/hour"),
        "user": env("THROTTLE_USER_RATE", default="1000/hour"),
    },
    "DEFAULT_FILTER_BACKENDS": ["django_filters.rest_framework.DjangoFilterBackend"],
    "EXCEPTION_HANDLER": "config.exception_handler.custom_exception_handler",
}

Environment-specific files extend base:

# config/settings/local.py
from config.settings.base import *  # noqa: F403

DEBUG = True
ALLOWED_HOSTS = ["*"]

# config/settings/test.py
from config.settings.base import *  # noqa: F403

DATABASES = {"default": env.db("TEST_DATABASE_URL", default="sqlite://:memory:")}
PASSWORD_HASHERS = ["django.contrib.auth.hashers.MD5PasswordHasher"]

Throttling & Filtering

Use django-filter with FilterSet classes. Configure rates in settings — never per-view.

# apps/orders/filters.py
import django_filters

from apps.orders.models import Order


class OrderFilter(django_filters.FilterSet):
    status = django_filters.ChoiceFilter(choices=Order.Status.choices)
    created_after = django_filters.DateTimeFilter(field_name="created_at", lookup_expr="gte")
    created_before = django_filters.DateTimeFilter(field_name="created_at", lookup_expr="lte")

    class Meta:
        model = Order
        fields = ["status", "created_after", "created_before"]

Attach to the ViewSet via filterset_class = OrderFilter — DRF applies it automatically.

Testing DRF

Use pytest-django with APIClient for functional tests and factory_boy for test data.

# apps/orders/tests/test_views.py
import pytest
from django.urls import reverse
from rest_framework import status
from rest_framework.test import APIClient

from apps.orders.tests.factories import OrderFactory, UserFactory


@pytest.fixture
def api_client() -> APIClient:
    return APIClient()


@pytest.fixture
def auth_client(api_client: APIClient) -> APIClient:
    user = UserFactory()
    api_client.force_authenticate(user=user)
    api_client.user = user  # type: ignore[attr-defined]
    return api_client


@pytest.mark.django_db
class TestOrderViewSet:
    def test_list_returns_only_own_orders(self, auth_client: APIClient) -> None:
        own_order = OrderFactory(user=auth_client.user)
        OrderFactory()  # belongs to another user

        response = auth_client.get(reverse("orders:order-list"))

        assert response.status_code == status.HTTP_200_OK
        assert len(response.data["results"]) == 1
        assert response.data["results"][0]["id"] == own_order.pk

    def test_create_order_returns_201(self, auth_client: APIClient) -> None:
        product = ProductFactory()
        payload = {"product": product.pk, "quantity": 2}

        response = auth_client.post(reverse("orders:order-list"), data=payload)

        assert response.status_code == status.HTTP_201_CREATED
        assert response.data["quantity"] == 2

    def test_cancel_action_returns_200(self, auth_client: APIClient) -> None:
        order = OrderFactory(user=auth_client.user, status=Order.Status.PENDING)

        response = auth_client.post(reverse("orders:order-cancel", args=[order.pk]))

        assert response.status_code == status.HTTP_200_OK
        order.refresh_from_db()
        assert order.status == Order.Status.CANCELLED

    def test_unauthenticated_returns_403(self, api_client: APIClient) -> None:
        response = api_client.get(reverse("orders:order-list"))
        assert response.status_code == status.HTTP_403_FORBIDDEN

Use RequestFactory for unit-testing a single view in isolation (no middleware, no URL routing):

from django.test import RequestFactory
from apps.orders.views import OrderViewSet
from apps.orders.tests.factories import UserFactory

def test_get_queryset_filters_by_user() -> None:
    user = UserFactory.build()
    request = RequestFactory().get("/")
    request.user = user
    view = OrderViewSet()
    view.request = request  # type: ignore[assignment]
    view.kwargs = {}
    qs = view.get_queryset()
    assert qs.query.where  # filter is applied

Conventions

• Use @pytest.mark.django_db on test classes — pytest-django handles transactions and rollback.
• Use APIClient.force_authenticate() for auth — never create real tokens in unit tests.
• Use factory_boy Factory classes for all test data — never use Model.objects.create() directly in tests.
• Test the HTTP interface (status codes, response shape) for view tests; test domain logic in test_models.py / test_services.py.
• See the python-tdd-workflow skill for full testing rules and factory patterns.