Django Patterns

Django Patterns

Critical rules (non-obvious)

• N+1 queries: always use select_related (FK) / prefetch_related (M2M) — never iterate and query inside loops
• get_or_create race condition: wrap in transaction.atomic() in concurrent environments
• Never call save() inside pre_save signal — causes infinite recursion; use update_fields
• bulk_create skips signals and save() — don't use when signal logic is required
• Migrations on large tables: use RunSQL with CONCURRENTLY index creation to avoid locks

ORM: select_related vs prefetch_related

# FK / OneToOne → select_related (JOIN)
books = Book.objects.select_related("author", "author__publisher").all()

# M2M / reverse FK → prefetch_related (separate query)
authors = Author.objects.prefetch_related("books", "books__tags").all()

# Custom prefetch with queryset
from django.db.models import Prefetch
Author.objects.prefetch_related(
    Prefetch("books", queryset=Book.objects.filter(published=True), to_attr="active_books")
)

ORM: annotations and aggregations

from django.db.models import Count, Avg, Q, F, Value
from django.db.models.functions import Coalesce

Author.objects.annotate(
    book_count=Count("books"),
    avg_rating=Coalesce(Avg("books__rating"), Value(0.0)),
    high_rated=Count("books", filter=Q(books__rating__gte=4)),
).filter(book_count__gt=0).order_by("-book_count")

ORM: F expressions (avoid race conditions)

# BAD — race condition
product = Product.objects.get(pk=pk)
product.stock -= quantity
product.save()

# GOOD — atomic at DB level
Product.objects.filter(pk=pk).update(stock=F("stock") - quantity)

Views: class-based view pattern

from django.views import View
from django.contrib.auth.mixins import LoginRequiredMixin

class OrderDetailView(LoginRequiredMixin, View):
    def get(self, request, pk):
        order = get_object_or_404(Order.objects.select_related("user"), pk=pk, user=request.user)
        return JsonResponse(OrderSerializer(order).data)

DRF: serializer with validation

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ["id", "name", "price", "stock"]
        read_only_fields = ["id"]

    def validate_price(self, value):
        if value <= 0:
            raise serializers.ValidationError("Price must be positive.")
        return value

    def validate(self, data):  # cross-field
        if data["stock"] == 0 and data.get("is_featured"):
            raise serializers.ValidationError("Out-of-stock products cannot be featured.")
        return data

DRF: ViewSet with custom actions

from rest_framework import viewsets, status
from rest_framework.decorators import action
from rest_framework.response import Response

class ProductViewSet(viewsets.ModelViewSet):
    queryset = Product.objects.select_related("category")
    serializer_class = ProductSerializer
    permission_classes = [IsAuthenticated]

    @action(detail=True, methods=["post"], url_path="archive")
    def archive(self, request, pk=None):
        product = self.get_object()
        product.is_archived = True
        product.save(update_fields=["is_archived"])
        return Response(status=status.HTTP_204_NO_CONTENT)

DRF: filtering + pagination

# settings.py
REST_FRAMEWORK = {
    "DEFAULT_FILTER_BACKENDS": ["django_filters.rest_framework.DjangoFilterBackend"],
    "DEFAULT_PAGINATION_CLASS": "rest_framework.pagination.CursorPagination",
    "PAGE_SIZE": 20,
}

# viewset
class ProductViewSet(viewsets.ReadOnlyModelViewSet):
    filterset_fields = {"price": ["gte", "lte"], "category": ["exact"]}
    ordering_fields = ["price", "created_at"]
    search_fields = ["name", "description"]

Celery task pattern

from celery import shared_task
from django.db import transaction

@shared_task(bind=True, max_retries=3, default_retry_delay=60)
def send_order_email(self, order_id: int):
    try:
        order = Order.objects.select_related("user").get(pk=order_id)
        send_email(order.user.email, order)
    except Order.DoesNotExist:
        return  # don't retry if deleted
    except Exception as exc:
        raise self.retry(exc=exc)

# Dispatch after DB commit — avoids race condition
def create_order(data):
    with transaction.atomic():
        order = Order.objects.create(**data)
        transaction.on_commit(lambda: send_order_email.delay(order.pk))

Common pitfalls

Converted and distributed by TomeVault — claim your Tome and manage your conversions.