django-drf

What this skill does

## Critical Patterns

- ALWAYS separate serializers by operation: Read / Create / Update / Include
- ALWAYS use `filterset_class` for complex filtering (not `filterset_fields`)
- ALWAYS validate unknown fields in write serializers (inherit `BaseWriteSerializer`)
- ALWAYS use `select_related`/`prefetch_related` in `get_queryset()` to avoid N+1
- ALWAYS handle `swagger_fake_view` in `get_queryset()` for schema generation
- ALWAYS use `@extend_schema_field` for OpenAPI docs on `SerializerMethodField`
- NEVER put business logic in serializers - use services/utils
- NEVER use auto-increment PKs - use UUIDv4 or UUIDv7
- NEVER use trailing slashes in URLs (`trailing_slash=False`)

> **Note:** `swagger_fake_view` is specific to **drf-spectacular** for OpenAPI schema generation.

---

## Implementation Checklist

When implementing a new endpoint, review these patterns in order:

| # | Pattern | Reference | Key Points |
|---|---------|-----------|------------|
| 1 | **Models** | `api/models.py` | UUID PK, `inserted_at`/`updated_at`, `JSONAPIMeta.resource_name` |
| 2 | **ViewSets** | `api/base_views.py`, `api/v1/views.py` | Inherit `BaseRLSViewSet`, `get_queryset()` with N+1 prevention |
| 3 | **Serializers** | `api/v1/serializers.py` | Separate Read/Create/Update/Include, inherit `BaseWriteSerializer` |
| 4 | **Filters** | `api/filters.py` | Use `filterset_class`, inherit base filter classes |
| 5 | **Permissions** | `api/base_views.py` | `required_permissions`, `set_required_permissions()` |
| 6 | **Pagination** | `api/pagination.py` | Custom pagination class if needed |
| 7 | **URL Routing** | `api/v1/urls.py` | `trailing_slash=False`, kebab-case paths |
| 8 | **OpenAPI Schema** | `api/v1/views.py` | `@extend_schema_view` with drf-spectacular |
| 9 | **Tests** | `api/tests/test_views.py` | JSON:API content type, fixture patterns |

> **Full file paths**: See [references/file-locations.md](references/file-locations.md)

---

## Decision Trees

### Which Serializer?
```text
GET list/retrieve → <Model>Serializer
POST create       → <Model>CreateSerializer
PATCH update      → <Model>UpdateSerializer
?include=...      → <Model>IncludeSerializer
```

### Which Base Serializer?
```text
Read-only serializer   → BaseModelSerializerV1
Create with tenant_id  → RLSSerializer + BaseWriteSerializer (auto-injects tenant_id on create)
Update with validation → BaseWriteSerializer (tenant_id already exists on object)
Non-model data         → BaseSerializerV1
```

### Which Filter Base?
```text
Direct FK to Provider  → BaseProviderFilter
FK via Scan           → BaseScanProviderFilter
No provider relation  → FilterSet
```

### Which Base ViewSet?
```text
RLS-protected model  → BaseRLSViewSet (most common)
Tenant operations    → BaseTenantViewset
User operations      → BaseUserViewset
No RLS required      → BaseViewSet (rare)
```

### Resource Name Format?
```text
Single word model     → plural lowercase           (Provider → providers)
Multi-word model      → plural lowercase kebab     (ProviderGroup → provider-groups)
Through/join model    → parent-child pattern       (UserRoleRelationship → user-roles)
Aggregation/overview  → descriptive kebab plural   (ComplianceOverview → compliance-overviews)
```

---

## Serializer Patterns

### Base Class Hierarchy

```python
# Read serializer (most common)
class ProviderSerializer(RLSSerializer):
    class Meta:
        model = Provider
        fields = ["id", "provider", "uid", "alias", "connected", "inserted_at"]

# Write serializer (validates unknown fields)
class ProviderCreateSerializer(RLSSerializer, BaseWriteSerializer):
    class Meta:
        model = Provider
        fields = ["provider", "uid", "alias"]

# Include serializer (sparse fields for ?include=)
class ProviderIncludeSerializer(RLSSerializer):
    class Meta:
        model = Provider
        fields = ["id", "alias"]  # Minimal fields
```

### SerializerMethodField with OpenAPI

```python
from drf_spectacular.utils import extend_schema_field

class ProviderSerializer(RLSSerializer):
    connection = serializers.SerializerMethodField(read_only=True)

    @extend_schema_field({
        "type": "object",
        "properties": {
            "connected": {"type": "boolean"},
            "last_checked_at": {"type": "string", "format": "date-time"},
        },
    })
    def get_connection(self, obj):
        return {
            "connected": obj.connected,
            "last_checked_at": obj.connection_last_checked_at,
        }
```

### Included Serializers (JSON:API)

```python
class ScanSerializer(RLSSerializer):
    included_serializers = {
        "provider": "api.v1.serializers.ProviderIncludeSerializer",
    }
```

### Sensitive Data Masking

```python
def to_representation(self, instance):
    data = super().to_representation(instance)
    # Mask by default, expose only on explicit request
    fields_param = self.context.get("request").query_params.get("fields[my-model]", "")
    if "api_key" in fields_param:
        data["api_key"] = instance.api_key_decoded
    else:
        data["api_key"] = "****" if instance.api_key else None
    return data
```

---

## ViewSet Patterns

### get_queryset() with N+1 Prevention

**Always combine** `swagger_fake_view` check with `select_related`/`prefetch_related`:

```python
def get_queryset(self):
    # REQUIRED: Return empty queryset for OpenAPI schema generation
    if getattr(self, "swagger_fake_view", False):
        return Provider.objects.none()

    # N+1 prevention: eager load relationships
    return Provider.objects.select_related(
        "tenant",
    ).prefetch_related(
        "provider_groups",
        Prefetch("tags", queryset=ProviderTag.objects.filter(tenant_id=self.request.tenant_id)),
    )
```

> **Why swagger_fake_view?** drf-spectacular introspects ViewSets to generate OpenAPI schemas. Without this check, it executes real queries and can fail without request context.

### Action-Specific Serializers

```python
def get_serializer_class(self):
    if self.action == "create":
        return ProviderCreateSerializer
    elif self.action == "partial_update":
        return ProviderUpdateSerializer
    elif self.action in ["connection", "destroy"]:
        return TaskSerializer
    return ProviderSerializer
```

### Dynamic Permissions per Action

```python
class ProviderViewSet(BaseRLSViewSet):
    required_permissions = [Permissions.MANAGE_PROVIDERS]

    def set_required_permissions(self):
        if self.action in ["list", "retrieve"]:
            self.required_permissions = []  # Read-only = no permission
        else:
            self.required_permissions = [Permissions.MANAGE_PROVIDERS]
```

### Cache Decorator

```python
from django.utils.decorators import method_decorator
from django.views.decorators.cache import cache_control

CACHE_DECORATOR = cache_control(
    max_age=django_settings.CACHE_MAX_AGE,
    stale_while_revalidate=django_settings.CACHE_STALE_WHILE_REVALIDATE,
)

@method_decorator(CACHE_DECORATOR, name="list")
@method_decorator(CACHE_DECORATOR, name="retrieve")
class ProviderViewSet(BaseRLSViewSet):
    pass
```

### Custom Actions

```python
# Detail action (operates on single object)
@action(detail=True, methods=["post"], url_name="connection")
def connection(self, request, pk=None):
    instance = self.get_object()
    # Process instance...

# List action (operates on collection)
@action(detail=False, methods=["get"], url_name="metadata")
def metadata(self, request):
    queryset = self.filter_queryset(self.get_queryset())
    # Aggregate over queryset...
```

---

## Filter Patterns

### Base Filter Classes

```python
class BaseProviderFilter(FilterSet):
    """For models with direct FK to Provider"""
    provider_id = UUIDFilter(field_name="provider__id", lookup_expr="exact")
    provider_id__in = UUIDInFilter(field_name="provider__id", lookup_expr="in")
    provider_type = ChoiceFilter(field_name="provider__provider", choices=Provider.ProviderChoices.choices)

class BaseScanProvider