Graphene-Django更新Mutation报错：String无法表示元组值

你在编写Graphene更新Mutation时遇到了字段被设置为元组的错误，导致返回数据格式异常并报错。

问题代码 #

class UpdateAddress(graphene.Mutation):
    address = graphene.Field(AddressType)
    
    class Arguments:
        address_id = graphene.ID(required=True)
        index = graphene.String()
        street = graphene.String()
        house = graphene.String()
        flat = graphene.String()
        locality = graphene.Int()
        
    @classmethod
    def mutate(cls, root, info, address_id, index=None, street=None, house=None, flat=None, locality=None):
        address = Address.objects.get(pk=address_id)
        try:
            entry = Locality.objects.get(pk=locality)
        except Locality.DoesNotExist:
            entry = None

        address.index=index,
        address.street=street,
        address.house=house,
        address.flat=flat,
        address.locality=entry

        address.save()
        return UpdateAddress(address=address)

执行的更新查询 #

mutation {
  updateAddress(
    addressId: "1",
    index: "41204"
    street: "Shevchenko",
    house: "1",
    flat: "15",
    locality: 1
  ) {
    address{
      id,
      street,
      index,
      house,
      flat,
      locality{
        id
      }
    }
  }
}

报错信息 #

"errors": [
    {
      "message": "String cannot represent value: ('41204',)",
      "locations": [
        {
          "line": 12,
          "column": 7
        }
      ],
      "path": [
        "updateAddress",
        "address",
        "index"
      ]
    }
  ]

问题原因 #

你在给address对象的字段赋值时，每一行末尾都加了逗号，比如address.index=index,，这会让Python把赋值的右侧解析成一个单元素元组（(index,)），而不是直接赋值index本身。所以最终数据库里的字符串字段被存储成了元组的字符串表示形式，导致Graphene无法正确序列化。

修正后的代码 #

只需要去掉赋值语句末尾的逗号即可：

class UpdateAddress(graphene.Mutation):
    address = graphene.Field(AddressType)
    
    class Arguments:
        address_id = graphene.ID(required=True)
        index = graphene.String()
        street = graphene.String()
        house = graphene.String()
        flat = graphene.String()
        locality = graphene.Int()
        
    @classmethod
    def mutate(cls, root, info, address_id, index=None, street=None, house=None, flat=None, locality=None):
        address = Address.objects.get(pk=address_id)
        try:
            entry = Locality.objects.get(pk=locality)
        except Locality.DoesNotExist:
            entry = None

        # 去掉每一行末尾的逗号
        address.index = index
        address.street = street
        address.house = house
        address.flat = flat
        address.locality = entry

        address.save()
        return UpdateAddress(address=address)

额外优化建议 #

如果不想逐个字段赋值，可以用kwargs简化代码（注意过滤掉address_id和locality这些需要特殊处理的参数）：

@classmethod
def mutate(cls, root, info, address_id, locality=None, **kwargs):
    address = Address.objects.get(pk=address_id)
    if locality is not None:
        try:
            address.locality = Locality.objects.get(pk=locality)
        except Locality.DoesNotExist:
            pass
    # 批量更新其他字段
    for field, value in kwargs.items():
        if value is not None:
            setattr(address, field, value)
    address.save()
    return UpdateAddress(address=address)

内容的提问来源于stack exchange，提问作者AndyFlight