Quick Reference

Critical Rules

• QuerySets are lazy — iterating twice hits DB twice, use list() to cache
• select_related for FK/O2O, prefetch_related for M2M — or N+1 queries
• update() skips save() — no signals fire, no auto_now update
• F() for atomic updates — F('count') + 1 avoids race conditions
• get() raises DoesNotExist or MultipleObjectsReturned — use filter().first() for safe
• DEBUG=False requires ALLOWED_HOSTS — 400 Bad Request without it
• Forms need {% csrf_token %} — or 403 Forbidden on POST
• auto_now can't be overridden — use default=timezone.now if need manual set
• exclude(field=None) excludes NULL — use filter(field__isnull=True) for NULL
• Circular imports in models — use string reference: ForeignKey('app.Model')
• transaction.atomic() doesn't catch exceptions — errors still propagate
• sync_to_async for ORM in async views — ORM is sync-only