Django @ transaction.atomic()阻止在并发中创建对象

Question

我有一个票模型,它的票序列化器.票证模型有一个bought和一个booked_at字段.还有unique_togethershow和seat 的属性.

class Ticket(models.Model):
    show = models.ForeignKey(Show, on_delete=models.CASCADE)
    seat = models.ForeignKey(Seat, on_delete=models.CASCADE)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    booked_at = models.DateTimeField(default=timezone.now)
    bought = models.BooleanField(default=False)

    class Meta:
        unique_together = ('show', 'seat')

• 在故障单序列化程序中,验证时序列化程序会检查是否有任何具有所需席位和显示的故障单
  • 如果有票,则检查票是否被买入.
    • 如果它被购买,那么它将引发错误.
    • 如果没有购买,请检查是否在5分钟内预订了机票.
      • 如果在5分钟内预订,则引发错误.
      • 否则,如果预订时间超过5分钟,则删除旧票并返回有效.
• 如果没有票,则返回有效

TicketSerializer:

class TicketSerializer(serializers.Serializer):
    seat = serializers.PrimaryKeyRelatedField(queryset=Seat.objects.all())
    show = serializers.PrimaryKeyRelatedField(queryset=Show.objects.all())
    user = serializers.PrimaryKeyRelatedField(queryset=User.objects.all())
    bought = serializers.BooleanField(default=False)

    def validate(self, attrs):
        if attrs['seat']:
            try:
                ticket = Ticket.objects.get(show=attrs['show'], seat=seat)
                if not ticket.bought:
                    if ticket.booked_at < timezone.now() - datetime.timedelta(minutes=5):
                        # ticket booked crossed the deadline
                        ticket.delete()
                        return attrs
                    else:
                        # ticket in 5 mins range
                        raise serializers.ValidationError("Ticket with same show and seat exists.")
                else:
                    raise serializers.ValidationError("Ticket with same show and seat exists.")
            except Ticket.DoesNotExist:
                return attrs
        else:
            raise serializers.ValidationError("No seat value provided.")

在视图中,我@transaction.atomic()用来确保仅当所有票证都有效时才创建票证,或者如果无效则不创建任何票证.

@transaction.atomic()
@list_route(
    methods=['POST'],
    permission_classes=[IsAuthenticated],
    url_path='book-tickets-by-show/(?P<show_id>[0-9]+)'
)
def book_tickets_by_show(self, request, show_id=None):
    try:
        show = Show.objects.get(id=show_id)
        user = request.user
        ...
        ...
        data_list = [...]
        with transaction.atomic():
            try:
                serializer = TicketSerializer(data=data_list, many=True)
                if serializer.is_valid():
                    serializer.save()
                    ....
                return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
            except (Seat.DoesNotExist, ValueError, ConnectionError) as e:
                return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)
    except (Show.DoesNotExist, IntegrityError) as e:
        return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)

我想知道的是,它有助于防止在为同一个席位创建多个请求时创建机票/ s？

假设,用户A想要预订座位5,6的票.用户B想要预订座位3,6的票,而另一个用户C想要预订座位2,3,4,5,6的票.

上述方法是否会阻止为所有用户预订各自座位的票证,并且只为一个用户创建票证(可能是谁的交易是第一个)？或者,如果有更好的方法,那么请你告诉我如何.我希望我很清楚.如果没有,请问.

Answer 1

它是否有助于防止调用多个请求来为同一座位创建门票？

是的，它会。unique_together加号约束将transaction.atomic()确保您无法为同一座位/演出创建两张门票。

也就是说，您当前的方法存在几个问题：

1. 我认为没有必要包装整个视图以及进行节省的部分atomic()- 您不需要同时执行这两项操作，并且将整个视图包装在事务中会带来性能成本。将其包装serializer.save()在事务中应该就足够了。

2. 不建议在事务中捕获异常 - 请参阅文档中的警告。通常，最好在尽可能接近生成异常的代码处捕获异常，以避免混淆。我建议将代码重构为这样的内容。

   try:
       show = Show.objects.get(id=show_id)
   # Catch this specific exception where it happens, rather than at the bottom.
   except Show.DoesNotExist as e:
       return Response({'detail': str(e)}
   
   user = request.user
   ...
   ...
   data_list = [...]
   
   try:
       serializer = TicketSerializer(data=data_list, many=True)
       if serializer.is_valid():
           try:
               # Note - this is now *inside* a try block, not outside
               with transaction.atomic():
                   serializer.save()
                   ....
           except IntegrityError as e:
               return Response({'detail': str(e), status=status.HTTP_400_BAD_REQUEST}
   
       return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
   # Retained from your code - althought I am not sure how you would 
   # end up with ever get a Seat.DoesNotExist or ValueError error here
   # Would be better to catch them in the place they can occur.
   except (Seat.DoesNotExist, ValueError, ConnectionError) as e:
       return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)