Agent Skills: Read serializer (most common)

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

Decision Trees

Which Serializer?

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

Which Base Serializer?

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?

Direct FK to Provider  → BaseProviderFilter
FK via Scan           → BaseScanProviderFilter
No provider relation  → FilterSet

Which Base ViewSet?

RLS-protected model  → BaseRLSViewSet (most common)
Tenant operations    → BaseTenantViewset
User operations      → BaseUserViewset
No RLS required      → BaseViewSet (rare)

Resource Name Format?

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

# 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

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)

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

Sensitive Data Masking

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:

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

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

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

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

# 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

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 BaseScanProviderFilter(FilterSet):
    """For models with FK to Scan (Scan has FK to Provider)"""
    provider_id = UUIDFilter(field_name="scan__provider__id", lookup_expr="exact")

Custom Multi-Value Filters

class UUIDInFilter(BaseInFilter, UUIDFilter):
    pass

class CharInFilter(BaseInFilter, CharFilter):
    pass

class ChoiceInFilter(BaseInFilter, ChoiceFilter):
    pass

ArrayField Filtering

# Single value contains
region = CharFilter(method="filter_region")

def filter_region(self, queryset, name, value):
    return queryset.filter(resource_regions__contains=[value])

# Multi-value overlap
region__in = CharInFilter(field_name="resource_regions", lookup_expr="overlap")

Date Range Validation

def filter_queryset(self, queryset):
    # Require date filter for performance
    if not (date_filters_provided):
        raise ValidationError([{
            "detail": "At least one date filter is required",
            "status": 400,
            "source": {"pointer": "/data/attributes/inserted_at"},
            "code": "required",
        }])

    # Validate max range
    if date_range > settings.FINDINGS_MAX_DAYS_IN_RANGE:
        raise ValidationError(...)

    return super().filter_queryset(queryset)

Dynamic FilterSet Selection

def get_filterset_class(self):
    if self.action in ["latest", "metadata_latest"]:
        return LatestFindingFilter
    return FindingFilter

Enum Field Override

class Meta:
    model = Finding
    filter_overrides = {
        FindingDeltaEnumField: {"filter_class": CharFilter},
        StatusEnumField: {"filter_class": CharFilter},
        SeverityEnumField: {"filter_class": CharFilter},
    }

Performance Patterns

PaginateByPkMixin

For large querysets with expensive joins:

class PaginateByPkMixin:
    def paginate_by_pk(self, request, base_queryset, manager,
                       select_related=None, prefetch_related=None):
        # 1. Get PKs only (cheap)
        pk_list = base_queryset.values_list("id", flat=True)
        page = self.paginate_queryset(pk_list)

        # 2. Fetch full objects for just the page
        queryset = manager.filter(id__in=page)
        if select_related:
            queryset = queryset.select_related(*select_related)
        if prefetch_related:
            queryset = queryset.prefetch_related(*prefetch_related)

        # 3. Re-sort to preserve DB ordering
        queryset = sorted(queryset, key=lambda obj: page.index(obj.id))
        return self.get_paginated_response(self.get_serializer(queryset, many=True).data)

Prefetch in Serializers

def get_tags(self, obj):
    # Use prefetched tags if available
    if hasattr(obj, "prefetched_tags"):
        return {tag.key: tag.value for tag in obj.prefetched_tags}
    # Fallback (causes N+1 if not prefetched)
    return obj.get_tags(self.context.get("tenant_id"))

Naming Conventions

| Entity | Pattern | Example | |--------|---------|---------| | Serializer (read) | <Model>Serializer | ProviderSerializer | | Serializer (create) | <Model>CreateSerializer | ProviderCreateSerializer | | Serializer (update) | <Model>UpdateSerializer | ProviderUpdateSerializer | | Serializer (include) | <Model>IncludeSerializer | ProviderIncludeSerializer | | Filter | <Model>Filter | ProviderFilter | | ViewSet | <Model>ViewSet | ProviderViewSet |

OpenAPI Documentation

from drf_spectacular.utils import extend_schema, extend_schema_view

@extend_schema_view(
    list=extend_schema(tags=["Provider"], summary="List all providers"),
    retrieve=extend_schema(tags=["Provider"], summary="Retrieve provider"),
    create=extend_schema(tags=["Provider"], summary="Create provider"),
)
@extend_schema(tags=["Provider"])
class ProviderViewSet(BaseRLSViewSet):
    pass

API Security Patterns

Full examples: See assets/security_patterns.py

| Pattern | Key Points | |---------|------------| | Input Validation | Use validate_<field>() for sanitization, validate() for cross-field | | Prevent Mass Assignment | ALWAYS use explicit fields list, NEVER __all__ or exclude | | Object-Level Permissions | Implement has_object_permission() for ownership checks | | Rate Limiting | Configure DEFAULT_THROTTLE_RATES, use per-view throttles for sensitive endpoints | | Prevent Info Disclosure | Generic error messages, return 404 not 403 for unauthorized (prevents enumeration) | | SQL Injection | ALWAYS use ORM parameterization, NEVER string interpolation in raw SQL |

Quick Reference

# Input validation in serializer
def validate_uid(self, value):
    value = value.strip().lower()
    if not re.match(r'^[a-z0-9-]+$', value):
        raise serializers.ValidationError("Invalid format")
    return value

# Explicit fields (prevent mass assignment)
class Meta:
    fields = ["name", "email"]  # GOOD: whitelist
    read_only_fields = ["id", "inserted_at"]  # System fields

# Object permission
class IsOwnerOrReadOnly(BasePermission):
    def has_object_permission(self, request, view, obj):
        if request.method in SAFE_METHODS:
            return True
        return obj.owner == request.user

# Throttling for sensitive endpoints
class BurstRateThrottle(UserRateThrottle):
    rate = "10/minute"

# Safe error messages (prevent enumeration)
def get_object(self):
    try:
        return super().get_object()
    except Http404:
        raise NotFound("Resource not found")  # Generic, no internal IDs

Commands

# Development
cd api && poetry run python src/backend/manage.py runserver
cd api && poetry run python src/backend/manage.py shell

# Database
cd api && poetry run python src/backend/manage.py makemigrations
cd api && poetry run python src/backend/manage.py migrate

# Testing
cd api && poetry run pytest -x --tb=short
cd api && poetry run make lint

Resources

Local References

• File Locations: See references/file-locations.md
• JSON:API Conventions: See references/json-api-conventions.md
• Security Patterns: See assets/security_patterns.py

Context7 MCP (Recommended)

Prerequisite: Install Context7 MCP server for up-to-date documentation lookup.

When implementing or debugging, query these libraries via mcp_context7_query-docs:

| Library | Context7 ID | Use For | |---------|-------------|---------| | Django | /websites/djangoproject_en_5_2 | Models, ORM, migrations | | DRF | /websites/django-rest-framework | ViewSets, serializers, permissions | | drf-spectacular | /tfranzel/drf-spectacular | OpenAPI schema, @extend_schema |

Example queries:

mcp_context7_query-docs(libraryId="/websites/django-rest-framework", query="ViewSet get_queryset best practices")
mcp_context7_query-docs(libraryId="/tfranzel/drf-spectacular", query="extend_schema examples for custom actions")
mcp_context7_query-docs(libraryId="/websites/djangoproject_en_5_2", query="model constraints and indexes")

Note: Use mcp_context7_resolve-library-id first if you need to find the correct library ID.

External Docs

• DRF Docs: https://www.django-rest-framework.org/
• DRF JSON:API: https://django-rest-framework-json-api.readthedocs.io/
• drf-spectacular: https://drf-spectacular.readthedocs.io/
• django-filter: https://django-filter.readthedocs.io/