Django保存模型报错：'id'期望数字却得到ForeignKey对象

错误信息：

Exception Type: TypeError
Exception Value: Field 'id' expected a number but got <django.db.models.fields.related.ForeignKey: owner>.

模型代码 #

class User(AbstractUser):
    pass

""" ... """

class Item(TimeStampMixin):
    title = models.CharField(max_length=40)
    owner = models.ForeignKey(User, on_delete=models.CASCADE, related_name="Items")
    last_bidder = models.ForeignKey(User, on_delete=models.DO_NOTHING, related_name="bidded_objects", default=owner)
    etc=etc...

视图代码 #

class Form_New_Item(ModelForm):
    class Meta:
        model = Item
        fields = ['title', 'description', 'price', 'category', 'image_url']

def create_new_item(request):
    if request.user.is_authenticated and request.method == "POST":
        form = Form_New_Item(request.POST)
        user = request.user
        if form.is_valid():
            new_item = Item(
                owner=user,
                title=form.cleaned_data['title'],
                etc=etc...,
            )
            new_item.save()
            return HttpResponseRedirect(reverse("index"))

错误根源在last_bidder字段的default设置：你直接把owner字段对象作为默认值，而不是一个有效的用户实例或用户ID。Django尝试将这个字段对象转换为用户ID时，就会抛出"Field 'id' expected a number but got..."的错误。

修复方法有两种：

• 如果允许last_bidder为空，设置default=None并添加null=True：

last_bidder = models.ForeignKey(User, on_delete=models.DO_NOTHING, related_name="bidded_objects", default=None, null=True)

• 如果希望默认值和owner保持一致，有两种实现方式：
  1. 在创建实例时显式赋值：

  new_item = Item(
      owner=user,
      last_bidder=user,
      title=form.cleaned_data['title'],
      etc=etc...,
  )
  

  2. 使用可调用函数动态获取默认值：

  def get_default_last_bidder(instance):
      return instance.owner
  
  class Item(TimeStampMixin):
      title = models.CharField(max_length=40)
      owner = models.ForeignKey(User, on_delete=models.CASCADE, related_name="Items")
      last_bidder = models.ForeignKey(User, on_delete=models.DO_NOTHING, related_name="bidded_objects", default=get_default_last_bidder)
  

  注意：第二种方式需要确保创建Item实例时owner已被正确设置，否则可能引发错误。

内容的提问来源于stack exchange，提问作者Santiago