config/mixins.py

"""Reusable DRF viewset mixins.

`IntegrityConflictMixin` converts a DB-level uniqueness *race* into a clean
409. It is not the first line of defense: DRF's auto-generated
`UniqueValidator` / `UniqueTogetherValidator` already reject the common
(non-concurrent) duplicate with a 400 during `is_valid()`, before the DB is
touched. This mixin only catches the genuine concurrent race that slips past
validation (two requests both pass `is_valid`, both reach the INSERT) — which
would otherwise surface as an uncaught 500.
"""
from django.db import IntegrityError, transaction
from rest_framework import status
from rest_framework.response import Response


class IntegrityConflictMixin:
    """Mix into a ModelViewSet to turn an INSERT-time IntegrityError into 409.

    The create is wrapped in its own `atomic()` block so the catch is
    savepoint-safe regardless of `ATOMIC_REQUESTS`: the failed INSERT rolls
    back cleanly and the `except` runs outside the (now-closed) block, so no
    later query hits a poisoned transaction.

    Set `conflict_detail` per viewset for a user-facing message.
    """

    conflict_detail = 'This record already exists.'

    def create(self, request, *args, **kwargs):
        try:
            with transaction.atomic():
                return super().create(request, *args, **kwargs)
        except IntegrityError:
            return Response(
                {'detail': self.conflict_detail},
                status=status.HTTP_409_CONFLICT,
            )