Django DRF Patterns

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.

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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

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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

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Commands

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

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

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

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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/