Django/Heroku：API数据写入数据库时的ObjectDoesNotExist异常处理

问题背景 #

我的Django应用从API接收数据，虽然API发送错误信息的概率极低，但需防范恶意请求引发的ObjectDoesNotExist异常。我尝试设置最多2次重试来处理该异常，但相关打印语句未触发，推测数据在初步验证时被判定为有效，仅在写入数据库阶段才发现对象不存在，现寻求可行的异常处理方案。

模型代码 #

class UserProfile(models.Model):
    user = models.OneToOneField(User, null=True, on_delete=models.CASCADE)

    def __str__(self):
        return str(self.user)

class ItemisedLoyaltyCard(models.Model):
    user = models.ForeignKey(UserProfile, blank=True, null=True, on_delete=models.CASCADE)
    venue = models.ForeignKey(Venue, blank=True, null=True, on_delete=models.CASCADE, related_name="venueitemisedloyaltycard")
    add_points = models.DecimalField(name = 'add_points', null = True, blank=True, default=0, decimal_places=2, max_digits=5)
    
    def __str__(self):
        return f"{self.user} - {self.venue}"  # 原代码未完成，此处补全以保证语法合法

处理函数代码 #

def API_data(request):
    userprofile_id = request.GET.get('userprofile_id', '')
    venue_id = request.GET.get('venue_id', '')

    user = UserProfile.objects.get(id=userprofile_id)
    venue = Venue.objects.get(id=venue_id)
    venue_loyalty_point_rule = VenueLoyaltyPointRule.objects.get(venue=venue)

    scans = ItemisedLoyaltyCard.objects.filter(user_id=user.id, venue=venue)
    is_point_valid = loyalty_card_validation_rule(scans, user, venue, venue_loyalty_point_rule)

    if is_point_valid:
        try_count = 0
        max_retries = 2  # 定义最大重试次数
        while try_count < max_retries:
            try:
                ItemisedLoyaltyCard.objects.create(
                    user_id=user.id, venue=venue, add_points=venue_loyalty_point_rule.points)
                # 创建成功则设置数据并跳出循环
                data = {'points': venue_loyalty_point_rule.points, 'user': user.id, 'venue': venue.name,
                        'success': is_point_valid, 'frequency_rule': venue_loyalty_point_rule.frequency}
                break
            except ObjectDoesNotExist:  # 此处打印未触发
                try_count += 1
                print("Retrying...")
        else:
            # 超过最大重试次数，设置错误信息
            print("Max retries exceeded. Unable to create loyalty card entry.")
            data = {'error': 'Max retries exceeded. Unable to create loyalty card entry.'}
    else:
        print("Invalid loyalty card validation rule.")
        data = {'error': 'Invalid loyalty card validation rule.'}

    return HttpResponse(json.dumps(data), content_type="application/json")

核心问题解析 #

1. 前置get操作未做异常处理：代码开头的UserProfile.objects.get()、Venue.objects.get()、VenueLoyaltyPointRule.objects.get()是最容易触发ObjectDoesNotExist的地方——恶意请求传入不存在的ID时，这些操作会直接抛出异常，根本不会走到后续的重试逻辑。
2. 重试逻辑捕获错误类型错误：ItemisedLoyaltyCard.objects.create()时，如果user或venue在之前get之后被删除（竞态条件），Django会抛出IntegrityError（数据库外键约束失败），而非ObjectDoesNotExist，这就是你的打印语句从未触发的原因。

可行解决方案 #

方案1：提前拦截无效请求，处理前置异常 #

首先在代码开头就对传入的ID做合法性校验，并捕获ObjectDoesNotExist，避免后续无效执行：

from django.core.exceptions import ObjectDoesNotExist
from django.http import JsonResponse

def API_data(request):
    userprofile_id = request.GET.get('userprofile_id', '')
    venue_id = request.GET.get('venue_id', '')

    # 先校验ID是否为有效数字
    if not (userprofile_id.isdigit() and venue_id.isdigit()):
        return JsonResponse({'error': 'Invalid ID format'}, status=400)

    try:
        user = UserProfile.objects.get(id=userprofile_id)
        venue = Venue.objects.get(id=venue_id)
        venue_loyalty_point_rule = VenueLoyaltyPointRule.objects.get(venue=venue)
    except ObjectDoesNotExist as e:
        print(f"Invalid request: {str(e)}")
        return JsonResponse({'error': 'Invalid user or venue ID'}, status=404)

    # 后续校验与创建逻辑...

方案2：修正重试逻辑，捕获正确异常并重新获取对象 #

针对竞态条件（对象在get后被删除），调整重试逻辑：捕获IntegrityError，且每次重试前重新查询对象，避免使用已失效的实例：

from django.db import IntegrityError

# ... 前置校验逻辑 ...

if is_point_valid:
    try_count = 0
    max_retries = 2
    data = {}
    while try_count < max_retries:
        try:
            # 重试时重新查询对象，确保实例存在
            fresh_user = UserProfile.objects.get(id=userprofile_id)
            fresh_venue = Venue.objects.get(id=venue_id)
            fresh_rule = VenueLoyaltyPointRule.objects.get(venue=fresh_venue)
            
            ItemisedLoyaltyCard.objects.create(
                user_id=fresh_user.id, venue=fresh_venue, add_points=fresh_rule.points)
            data = {'points': fresh_rule.points, 'user': fresh_user.id, 'venue': fresh_venue.name,
                    'success': is_point_valid, 'frequency_rule': fresh_rule.frequency}
            break
        except (ObjectDoesNotExist, IntegrityError) as e:
            try_count += 1
            print(f"Retry {try_count}/{max_retries}: {str(e)}")
    else:
        print("Max retries exceeded. Unable to create loyalty card entry.")
        data = {'error': 'Max retries exceeded. Unable to create loyalty card entry.'}

方案3：使用事务保证原子性 #

如果需要确保整个操作的原子性（比如验证和创建不能被其他操作打断），可以用Django的事务装饰器：

from django.db import transaction

@transaction.atomic
def create_loyalty_entry(user_id, venue_id):
    # 锁定查询对象，避免其他事务修改或删除
    user = UserProfile.objects.select_for_update().get(id=user_id)
    venue = Venue.objects.select_for_update().get(id=venue_id)
    rule = VenueLoyaltyPointRule.objects.get(venue=venue)
    ItemisedLoyaltyCard.objects.create(user_id=user.id, venue=venue, add_points=rule.points)
    return rule

# 在处理函数中调用：
if is_point_valid:
    try_count = 0
    max_retries = 2
    while try_count < max_retries:
        try:
            rule = create_loyalty_entry(userprofile_id, venue_id)
            data = {'points': rule.points, 'user': user.id, 'venue': venue.name,
                    'success': is_point_valid, 'frequency_rule': rule.frequency}
            break
        except (ObjectDoesNotExist, IntegrityError) as e:
            try_count += 1
            print(f"Retrying... {str(e)}")
    else:
        data = {'error': 'Max retries exceeded. Unable to create loyalty card entry.'}

select_for_update()会锁定查询到的对象，避免其他事务修改或删除，降低竞态条件的概率。

内容的提问来源于stack exchange，提问作者PhilM