Django一对多模型中Case的total_price字段更新异常求助

我是Django新手，在models.py中定义了Case和CaseItem两个模型，二者为一对多关系（一个Case可关联多个CaseItem）。我希望创建CaseItem并关联Case时，自动更新Case的total_price字段，但目前创建Case后该字段始终为默认值0。相关代码如下：

模型代码 #

class Item(models.Model):
    name = models.TextField(unique=True)
    # Price is in cents
    price = models.PositiveIntegerField(
        validators=[
            MinValueValidator(1)
        ]
    )
    img = models.TextField()


class Case(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid4)
    name = models.TextField(unique=True)
    case_img = models.TextField()
    total_price = models.PositiveIntegerField(default=0)

class CaseItem(models.Model):
    case = models.ForeignKey(Case, on_delete=models.CASCADE, related_name="items")
    item = models.ForeignKey(Item, on_delete=models.PROTECT)
    percentage = models.PositiveIntegerField(
        validators=[
            MaxValueValidator(100),
            MinValueValidator(1)
        ]
    )

    # ensure that user cant add the same item to the case
    class Meta:
        unique_together = [['case', 'item']]
    
    def save(self, *args, **kwargs):
        self.case.total_price += self.item.price
        self.case.save()
        super().save(*args, **kwargs)

创建视图代码 #

class CreateCustomCase(generics.CreateAPIView):
    serializer_class = CustomCaseSerializer

    def create(self, request, *args, **kwargs):
        data = request.data.copy()
        serializer = self.get_serializer(data=data, context={'request': request})

        if serializer.is_valid(raise_exception=True):
            serializer.save()
            return Response(serializer.validated_data, status.HTTP_201_CREATED)

        return Response(serializer.errors, status.HTTP_400_BAD_REQUEST)

序列化器代码 #

class CustomCaseSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=None)
    case_img = serializers.CharField(max_length=None)
    items = CustomCaseItemSerializer(many=True)

    def validate(self, attrs):
        items = attrs.get("items")
        case_name = attrs.get("name")

        try:
            caseObj = Case.objects.get(name=case_name)
        except Case.DoesNotExist:
            caseObj = None

        if caseObj is not None:
            raise serializers.ValidationError("Case with the same alredy exists")

        sumPercentage = sum([item["percentage"] for item in items])

        if sumPercentage != 100:
            raise serializers.ValidationError("Total Sum of percentage should be 100%")

        itemIdList = [item["id"] for item in items]
        selectIds = Item.objects.filter(pk__in=itemIdList).values('id')
        selectIdList = [selectId["id"] for selectId in selectIds]

        if len(selectIdList) != len(itemIdList):
            notInSelect = set(itemIdList).difference(selectIdList)
            raise serializers.ValidationError(
                f'Did not find the following items with id in the database: {notInSelect}'

1. 自定义序列化器缺失create方法：使用serializers.Serializer而非ModelSerializer时，必须手动实现create方法来处理Case和CaseItem的创建逻辑，否则默认save方法不会自动创建关联的CaseItem，自然无法触发CaseItem的save方法更新total_price。
2. CaseItem.save()逻辑存在漏洞：直接修改内存中的self.case对象累加价格，可能因数据库缓存导致数据不一致；后续若更新CaseItem关联的Item，还会出现重复累加的问题。
3. validate方法代码不完整：提供的validate方法缺少闭合的}，且未返回验证后的attrs，会导致代码报错或序列化器丢失有效数据。

1. 完善序列化器的create方法 #

在CustomCaseSerializer中添加create方法，先创建Case，再批量创建关联的CaseItem，确保触发CaseItem的save逻辑：

class CustomCaseSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=None)
    case_img = serializers.CharField(max_length=None)
    items = CustomCaseItemSerializer(many=True)

    # 补全并修正原validate方法
    def validate(self, attrs):
        items = attrs.get("items")
        case_name = attrs.get("name")

        try:
            Case.objects.get(name=case_name)
            raise serializers.ValidationError("Case with the same name already exists")
        except Case.DoesNotExist:
            pass

        sum_percentage = sum([item["percentage"] for item in items])
        if sum_percentage != 100:
            raise serializers.ValidationError("Total Sum of percentage should be 100%")

        item_id_list = [item["id"] for item in items]
        existing_item_ids = list(Item.objects.filter(pk__in=item_id_list).values_list('id', flat=True))
        missing_ids = set(item_id_list) - set(existing_item_ids)
        if missing_ids:
            raise serializers.ValidationError(
                f'Did not find the following items with id in the database: {missing_ids}'
            )
        return attrs

    def create(self, validated_data):
        # 提取items数据，先创建Case
        items_data = validated_data.pop('items')
        case = Case.objects.create(**validated_data)
        # 批量创建CaseItem，触发save方法更新total_price
        for item_data in items_data:
            item = Item.objects.get(pk=item_data['id'])
            CaseItem.objects.create(
                case=case,
                item=item,
                percentage=item_data['percentage']
            )
        # 刷新Case对象，获取最新的total_price
        case.refresh_from_db()
        return case

2. 修复CaseItem.save()逻辑 #

使用F表达式直接在数据库层面操作，避免内存对象与数据库数据不一致的问题，同时支持更新场景的价格修正：

class CaseItem(models.Model):
    # 保留原有字段和Meta类
    
    def save(self, *args, **kwargs):
        if self.pk is None:
            # 新建CaseItem时，累加对应Item的价格
            Case.objects.filter(pk=self.case.pk).update(
                total_price=models.F('total_price') + self.item.price
            )
        else:
            # 更新CaseItem时，先减去旧Item价格，再加新Item价格
            old_item = CaseItem.objects.only('item').get(pk=self.pk).item
            if old_item != self.item:
                Case.objects.filter(pk=self.case.pk).update(
                    total_price=models.F('total_price') - old_item.price + self.item.price
                )
        super().save(*args, **kwargs)

内容的提问来源于stack exchange，提问作者Ujjwal Sharma