dj-architecture

name	dj-architecture
description	Implement a Django feature following the opinionated architecture — prefixed ULID IDs, repository pattern, Pydantic DTOs, svcs service locator, project-scoped django-ninja API, Celery reliable signals, and layered tests. Use when the user asks to add a new entity, endpoint, app, or business logic in a Django project that follows these conventions.
allowed-tools	Read, Write, Edit, Bash, Grep, Glob

Implement a Django Feature

You are implementing a feature in an opinionated, fully type-safe Django project managed with uv. Every convention below is mandatory. Do not deviate.

Why this architecture exists: Django's ORM is powerful but hard to type — querysets, model instances, related managers, and F()/Q() expressions don't play well with static type checkers. This project solves that by pushing all ORM usage into repositories that return Pydantic DTOs. Services receive repos via constructor injection and contain pure business logic with zero ORM imports. Views are thin dispatchers. The result: everything from the repository boundary outward is fully typed, IDE-friendly, and testable in isolation.

Tooling: uv is the package manager. All commands use uv run. Never use pip, poetry, or raw python — always uv run python, uv run pytest, etc. To add a dependency: uv add <package>.

BEFORE WRITING CODE

Gather current project state by reading:

• src/project/ids.py — existing ID prefixes (must be unique)
• src/project/services.py — registered repos/services
• src/project/settings.py — INSTALLED_APPS
• src/project/api/__init__.py — NinjaAPI(), mounted resource routers, exception handlers
• src/project/api/<resource>/ — existing per-resource route packages
• src/project/types.py — AuthedRequest and other shared request types
• ARCHITECTURE.md if present — full pattern reference
• Any existing app the feature touches

Then state your implementation plan: models, DTOs, repos, services, routes, tests, and ID prefixes.

---

LAYER-BY-LAYER IMPLEMENTATION

Follow this exact order. Do not skip layers. Each layer has rules that are non-negotiable.

Layer 1: ID Generator

Add to src/project/ids.py:

generate_xxx_id = _make_generator("xxx")  # 3-4 char prefix

Prefixes must be unique across the project and short enough to be readable in logs.

Layer 2: Model

File: src/<app>/models/<entity>.py

Follow the models skill for full conventions. The key rules:

• class Meta is always first inside the model body — with verbose_name, verbose_name_plural, and indexes
• CharField(max_length=64) primary key with prefixed ULID default — NEVER UUIDs, NEVER auto-increment
• __prefix__: ClassVar[str] on every model
• All indexes in Meta.indexes — never db_index=True on fields. Optimize for queries the repository actually runs.
• ZERO business logic — no custom managers, no save() overrides, no signals, no properties that compute
• __str__ is the only method allowed

from typing import ClassVar
from django.db import models
from project.ids import generate_xxx_id

class MyEntity(models.Model):
    class Meta:
        verbose_name = "my entity"
        verbose_name_plural = "my entities"
        indexes = [
            models.Index(fields=["created_at"], name="idx_%(class)s_created"),
        ]

    __prefix__: ClassVar[str] = "xxx"
    id = models.CharField(max_length=64, primary_key=True, default=generate_xxx_id, editable=False)
    # fields...
    def __str__(self):
        return self.name

If this is a new app, add it to INSTALLED_APPS in src/project/settings.py using the dotted path to its AppConfig (e.g., "myapp.apps.MyAppConfig").

Then run:

uv run python src/manage.py makemigrations && uv run python src/manage.py migrate

Layer 3: DTO

File: src/<app>/dtos/<entity>.py

RULES:

• ALL ID fields are str — never UUID
• model_config = ConfigDict(from_attributes=True) — always
• For Django RelatedManager fields (reverse FKs, M2M), add the coercion validator:

@field_validator("children", mode="before")
@classmethod
def coerce_related_manager(cls, v):
    if hasattr(v, "all"):
        return list(v.all())
    return v

Layer 4: Repository

File: src/<app>/repositories/<entity>.py

RULES:

• ORM objects NEVER leave this layer — every public method returns a DTO or list[DTO]
• Convert with MyEntityDTO.model_validate(orm_obj)
• prefetch_related() when the DTO has nested relations
• @transaction.atomic on any method with multiple writes
• One repo per aggregate root — child entities are managed by the parent's repo
• All ID params are str

Layer 5: Service

File: src/<app>/services/<entity>.py

RULES:

• Receives repos via __init__ — NEVER instantiates them, NEVER imports models
• Contains all business logic: validation, orchestration, cross-repo coordination
• Touches ZERO ORM — no .objects, no F(), no Q(), no model imports
• Returns DTOs

Layer 6: Register in svcs

Add to src/project/services.py:

from myapp.repositories.my_entity import MyEntityRepository
from myapp.services.my_entity import MyEntityService

# Register the repository
registry.register_factory(MyEntityRepository, MyEntityRepository)

# Register the service with a factory that pulls its repo dependencies
def _my_entity_service_factory(container: svcs.Container) -> MyEntityService:
    repo = container.get(MyEntityRepository)
    return MyEntityService(repo)

registry.register_factory(MyEntityService, _my_entity_service_factory)

Both repos and services get registered. Service factories wire up repo dependencies via the container. The get() helper at the bottom of the file makes them available anywhere.

Layer 7: API Routes

Routes live in a per-resource package under src/project/api/ — NOT in app directories. Each resource is its own subpackage:

src/project/
└── api/
    ├── __init__.py          # NinjaAPI(), exception handlers, mounts routers
    ├── my_entities/
    │   ├── __init__.py      # re-exports `router` from routes
    │   ├── routes.py        # handlers
    │   └── schemas.py       # ninja.Schema input models
    └── other_resource/
        ├── __init__.py
        ├── routes.py
        └── schemas.py

RULES:

• Input schemas are ninja.Schema classes defined in schemas.py next to the routes
• Output schemas reuse the DTOs from the app — do not duplicate
• The resource package's __init__.py re-exports the router from routes (e.g. from .routes import router)
• The top-level src/project/api/__init__.py creates the NinjaAPI() instance, registers exception handlers, and mounts each resource router with api.add_router("/my-entities", my_entities_router)
• Pattern: from project.services import get, then get(MyEntityService) to obtain a wired service
• Every handler's first arg is typed request: AuthedRequest — never untyped. AuthedRequest lives in src/project/types.py and narrows request.user to an authenticated Django User
• ID path params are str
• Handlers do NOT try/except — errors bubble up to the central exception handler (see below)

src/project/api/my_entities/routes.py:

from typing import List

from ninja import Router, Status

from myapp.dtos.my_entity import MyEntityDTO
from myapp.services.my_entity import MyEntityService
from project.services import get
from project.types import AuthedRequest

from .schemas import CreateMyEntityIn

router = Router()


@router.get("/", response=List[MyEntityDTO])
def list_entities(request: AuthedRequest):
    return get(MyEntityService).list_entities()


@router.post("/", response={201: MyEntityDTO})
def create_entity(request: AuthedRequest, payload: CreateMyEntityIn):
    return Status(201, get(MyEntityService).create_entity(name=payload.name))


@router.get("/{entity_id}/", response=MyEntityDTO)
def get_entity(request: AuthedRequest, entity_id: str):
    return get(MyEntityService).get_entity(entity_id)

Central exception handlers

Services raise plain Python exceptions — ValueError for bad input, LookupError for missing records, PermissionError for forbidden access. They do NOT know about HTTP. The mapping happens once, centrally, in src/project/api/__init__.py:

• ValueError → 400 {"detail": str(exc)}
• LookupError → 404 {"detail": str(exc)}
• PermissionError → 403 {"detail": str(exc)}

@api.exception_handler(ValueError)
def on_value_error(request, exc: ValueError):
    return api.create_response(request, {"detail": str(exc)}, status=400)

Route handlers MUST NOT wrap service calls in try/except — errors bubble up and the central handler turns them into responses.

Layer 8: Admin

File: src/<app>/admin.py

Follow the models skill for full admin conventions. The key rules:

• Register every model with @admin.register
• list_display — id first, then 3-5 most useful columns
• list_per_page = 25 — keeps the admin fast on large tables
• search_fields — always include id, plus name/title fields
• readonly_fields — always include id (ULID PKs are never edited)
• ordering — explicit, usually -created_at or the primary time field
• list_select_related — specify FKs shown in list_display to avoid N+1s
• raw_id_fields or autocomplete_fields for FKs to large tables
• TabularInline for child models — extra = 0, show_change_link = True

---

TESTS

Write three test layers in tests/<app>/. No test file may be skipped.

test_repo.py — Real database, validate ORM ↔ DTO conversion

@pytest.mark.django_db
def test_create_and_get():
    repo = MyEntityRepository()
    dto = repo.create(name="Test", ...)
    assert isinstance(dto, MyEntityDTO)
    assert dto.id.startswith("xxx_")
    fetched = repo.get_by_id(dto.id)
    assert fetched == dto

@pytest.mark.django_db
def test_list_all():
    repo = MyEntityRepository()
    repo.create(name="A", ...)
    repo.create(name="B", ...)
    assert len(repo.list_all()) == 2

test_service.py — Mock the repos, validate business logic

Services are tested WITHOUT a database. Mock the repository. This is the most important test layer — it proves your business logic is correct independently of Django.

from unittest.mock import MagicMock

def test_create_delegates_to_repo():
    repo = MagicMock()
    expected = MyEntityDTO(id="xxx_fake", name="Test", ...)
    repo.create.return_value = expected

    service = MyEntityService(repo)
    result = service.create_entity(name="Test", ...)

    assert result == expected
    repo.create.assert_called_once_with(name="Test", ...)

def test_business_rule_rejects_bad_input():
    repo = MagicMock()
    # configure mock to trigger the rule
    service = MyEntityService(repo)

    with pytest.raises(ValueError, match="..."):
        service.do_something_invalid(...)

test_api.py — Integration through HTTP

@pytest.mark.django_db
def test_create(client):
    resp = client.post("/my-entities/", data={...}, content_type="application/json")
    assert resp.status_code == 201
    assert resp.json()["id"].startswith("xxx_")

@pytest.mark.django_db
def test_list_empty(client):
    resp = client.get("/my-entities/")
    assert resp.status_code == 200
    assert resp.json() == []

@pytest.mark.django_db
def test_get_by_id(client):
    create_resp = client.post("/my-entities/", data={...}, content_type="application/json")
    eid = create_resp.json()["id"]
    resp = client.get(f"/my-entities/{eid}/")
    assert resp.status_code == 200
    assert resp.json()["id"] == eid

---

RELIABLE SIGNALS (CELERY)

When a business operation needs to trigger async side-effects (notifications, cache invalidation, analytics), use reliable signals — NOT standard Django signals.

Signal Definition

File: src/<app>/signals.py

from project.signals import ReliableSignal

my_event = ReliableSignal()

Sending

Call send_reliable() inside a transaction.atomic() block in the service layer. Arguments MUST be JSON-serializable — pass entity IDs, never model instances:

# In the service method
def create_entity(self, name: str) -> MyEntityDTO:
    with transaction.atomic():
        entity = self.repo.create(name=name)
        my_event.send_reliable(sender=None, entity_id=entity.id)
    return entity

Receivers

File: src/<app>/receivers.py

Register with @receiver. Load receivers in apps.py → ready().

CRITICAL: Every receiver MUST be idempotent. The system guarantees at-least-once delivery, not exactly-once. A receiver may run more than once for the same event. Design accordingly:

• Check if the action was already performed before performing it
• Use database constraints or flags to prevent duplicate effects
• Never assume a receiver runs exactly once

from django.dispatch import receiver
from .signals import my_event

@receiver(my_event)
def on_my_event(obj_id: str, **kwargs):
    # Idempotent: guard against duplicate execution
    if already_processed(obj_id):
        return
    do_work(obj_id)

apps.py

class MyAppConfig(AppConfig):
    def ready(self):
        from . import receivers  # noqa: F401

Testing Receivers

Test receivers in isolation. Mock external dependencies. Verify idempotency by calling the receiver twice with the same arguments:

def test_receiver_is_idempotent():
    on_my_event(obj_id="xxx_fake")
    on_my_event(obj_id="xxx_fake")  # second call must be safe
    # assert side-effect happened exactly once

RULES

• NEVER use standard send() for post-commit side-effects — use send_reliable()
• Arguments MUST be JSON-serializable (strings, numbers, booleans)
• Receivers MUST be idempotent — this is non-negotiable
• Receivers MUST NOT import or touch ORM models directly — use a repository if DB access is needed

---

VERIFY

Run all four checks. ALL must pass before you report done.

uv run ruff check src
uv run ruff format --check src
uv run pyrefly check src
uv run pytest

If anything fails, fix it and re-run.

---

COMPLETION CHECKLIST

Before reporting done, confirm every item:

• ID generator in src/project/ids.py with unique 3-4 char prefix
• Model: Meta first (verbose names + indexes), __prefix__ ClassVar, CharField PK with ULID default, zero logic
• DTO: str IDs, from_attributes=True, RelatedManager coercion if needed
• Repository: returns DTOs only, model_validate(), @transaction.atomic for multi-writes
• Service: repos via __init__, zero ORM, business logic only
• Repo and service registered in src/project/services.py
• Routes in src/project/api/<resource>/routes.py, schemas in schemas.py, mounted in src/project/api/__init__.py using from project.services import get
• request: AuthedRequest annotation on every handler
• Central exception handlers registered in src/project/api/__init__.py (ValueError→400, LookupError→404, PermissionError→403)
• Admin registered per models skill conventions (list_display, list_per_page = 25, search_fields, readonly_fields, ordering, raw_id_fields/autocomplete_fields for large FKs, inlines with extra = 0)
• App in INSTALLED_APPS (if new) using dotted AppConfig path
• Migrations generated and applied
• test_repo.py: real DB, asserts ID prefix
• test_service.py: mocked repos, tests business logic
• test_api.py: HTTP integration, asserts status codes + response shape
• Signals in src/<app>/signals.py if async side-effects needed
• Receivers in src/<app>/receivers.py — idempotent, loaded in ready()
• ruff check, ruff format --check, pyrefly check, pytest all pass