Django Ninja框架外键赋值报错：需传入模型实例而非ID

问题现象 #

基于Django 4.1和Ninja 0.19.1开发的项目，通过Swagger或Postman发送POST请求时出现报错：

ValueError: Cannot assign "115": "Offer.currency_to_sell" must be a "Currency" instance.

提交的请求数据：

{
  "currency_to_sell_id": 115,
  "currency_to_buy_id": 116,
  "user_id": 1,
  "amount": 100,
  "exchange_rate": 10
}

相关代码：

api.py接口 #

@api.post("/add_offer/")
async def add_offer(request, payload: OfferIn):
    offer = await Offer.objects.acreate(**payload.dict())
    return {"id": offer.pk}

schemas.py定义 #

class OfferIn(ModelSchema):
    class Config:
        model = Offer
        model_fields = [
            "currency_to_sell",
            "currency_to_buy",
            "user",
            "amount",
            "exchange_rate",
        ]

Offer模型 #

class Offer(models.Model):
    """Sell currency offer model."""

    currency_to_sell = models.ForeignKey(
        to="Currency",
        on_delete=models.CASCADE,
        related_name="currencies_to_sell",
        verbose_name="Currency to sell",
    )
    currency_to_buy = models.ForeignKey(
        to="Currency",
        on_delete=models.CASCADE,
        related_name="currencies_to_buy",
        verbose_name="Currency to buy",
    )
    amount = models.DecimalField(
        decimal_places=2, max_digits=11, blank=False, null=False, verbose_name="Amount"
    )
    exchange_rate = models.DecimalField(
        decimal_places=2,
        max_digits=11,
        blank=False,
        null=False,
        verbose_name="Exchange rate",
    )
    user = models.ForeignKey(
        to=User, on_delete=models.CASCADE, related_name="offers", verbose_name="User"
    )

改用普通Schema而非ModelSchema时可正常运行：

class OfferIn(Schema):
    currency_to_sell_id: int = None
    currency_to_buy_id: int = None
    user_id: int = None
    amount: float
    exchange_rate: float

用户尝试了一种可行方法，但不确定是否正确：

@api.post("/offer/", tags=["Offer"])
async def add_offer(request, payload: OfferIn):

    currency_to_sell = await Currency.objects.aget(id=payload.currency_to_sell)
    currency_to_buy = await Currency.objects.aget(id=payload.currency_to_buy)
    user = await User.objects.aget(id=payload.user)
    payload.currency_to_sell = currency_to_sell
    payload.currency_to_buy = currency_to_buy
    payload.user = user
    offer = await Offer.objects.acreate(**payload.dict())
    return {"id": offer.pk}

错误原因 #

ModelSchema生成的OfferIn类，默认会将模型中的ForeignKey字段映射为接收对应模型实例的类型，但你提交的是xxx_id格式的纯数字ID。当直接把这些ID通过**payload.dict()传递给acreate时，Django ORM会尝试把数字赋值给需要模型实例的ForeignKey字段，从而触发报错。

而普通Schema中定义的xxx_id字段，Django ORM支持直接通过后缀_id的字段名赋值ForeignKey，因此可以正常运行。

解决方案 #

方案1：调整请求数据格式（推荐） #

Django Ninja的ModelSchema支持直接接收ForeignKey对应的ID值，只需将请求数据中的xxx_id字段名改为模型中定义的字段名（去掉_id后缀）：

{
  "currency_to_sell": 115,
  "currency_to_buy": 116,
  "user": 1,
  "amount": 100,
  "exchange_rate": 10
}

此时无需修改任何代码，原有的OfferIn和接口逻辑即可正常工作——Ninja会自动将ID转换为对应的模型实例。

方案2：修改ModelSchema，支持ID字段 #

如果必须保留xxx_id的请求字段格式，可以在OfferIn中显式定义ID字段，同时保留模型字段的映射：

class OfferIn(ModelSchema):
    currency_to_sell_id: int
    currency_to_buy_id: int
    user_id: int

    class Config:
        model = Offer
        model_fields = [
            "amount",
            "exchange_rate",
        ]

这样提交xxx_id格式的数据时，Schema会正确解析ID值，Django ORM也能通过xxx_id字段直接赋值ForeignKey。

方案3：手动转换实例（优化你的方法） #

你手动查询模型实例的方法是可行的，但建议添加异常处理，避免因ID不存在导致的崩溃，同时更清晰地传递参数：

from django.core.exceptions import ObjectDoesNotExist
from ninja import HttpError

@api.post("/offer/", tags=["Offer"])
async def add_offer(request, payload: OfferIn):
    try:
        currency_to_sell = await Currency.objects.aget(id=payload.currency_to_sell)
        currency_to_buy = await Currency.objects.aget(id=payload.currency_to_buy)
        user = await User.objects.aget(id=payload.user)
    except ObjectDoesNotExist as e:
        raise HttpError(404, str(e))
    
    offer = await Offer.objects.acreate(
        currency_to_sell=currency_to_sell,
        currency_to_buy=currency_to_buy,
        user=user,
        amount=payload.amount,
        exchange_rate=payload.exchange_rate
    )
    return {"id": offer.pk}

内容的提问来源于stack exchange，提问作者Vitalii Mytenko