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我的 typegraphql 代码有问题。我找不到导致此错误消息的原因。我在下面分享的代码是我的解析器和突变，它应该返回一个ProductResponse：

  @Mutation(returns => ProductResponse)
  async addProduct (@Args() { model, title, description, price, images, productCode }: productData):
  Promise<ProductResponse> {
    try {
      const productInstance = new ProductService()
      const resp = await productInstance.add(model, title, description, price, images, productCode)
      return { product: resp }
    } catch (err: any) {
      return { errorMessage: err.message }
    }
  }

下面是用于标准化所有突变和查询响应的ProductResponse类：

ObjectType()
export class ProductResponse {
  @Field(returns => String, { nullable: true })
  errorMessage?: string

  @Field(returns => Product, { nullable: true })
  product?: Product
}

这是我的服务，它从数据库返回保存的数据或引发错误：

  async add (model: string, title: string, description: string, price: number, images: string, productCode: string) {
    const product = await Product.findOne({ where: { productCode } })
    if (product) {
      throw new Error("Duplicate product")
    }

    const newProduct = await Product.create({ model, title, description, price, images, productCode }).save()

    return newProduct
  }

但是当我运行此代码时，我在下面遇到错误消息。我究竟做错了什么？

UnhandledPromiseRejectionWarning: Error: Cannot determine GraphQL output type for 'addProduct' of 'ProductResolver' class. Is the value, that is used as its TS type or explicit type, decorated with a proper decorator, or is it a proper output value?
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