Slow API Endpoint: N+1 Query Problem

# SLOW — N+1 queries (1 query for users + 1 query per user for orders)
class UserListView(APIView):
    def get(self, request):
        users = User.objects.all()   # Query 1: SELECT * FROM users
        result = []
        for user in users:
            result.append({
                "id": user.id,
                "name": user.name,
                "order_count": user.orders.count()  # Query 2...501: SELECT COUNT(*)
            })
        return Response(result)

# FAST — Single query with annotation
from django.db.models import Count

class UserListView(APIView):
    def get(self, request):
        users = User.objects.annotate(
            order_count=Count('orders')  # Single JOIN — one query total
        ).values('id', 'name', 'order_count')

        return Response(list(users))

# Alternatively, prefetch_related for complex nested data:
users = User.objects.prefetch_related('orders').all()
for user in users:
    # orders already loaded in 2 queries total, not N+1
    count = len(user.orders.all())

# Django Debug Toolbar or django-silk shows query count
from django.db import connection

def get_query_count():
    return len(connection.queries)

# Or log all queries during development
import logging
logging.getLogger('django.db.backends').setLevel(logging.DEBUG)