Apollo Federation模式，表示不完整

如何解决Apollo Federation模式，表示不完整

我有这个查询：

getMyTransactions {
  id
  createdAt
}

我得到了这个错误：

  "message": "Cannot return null for non-nullable field Transaction.createdAt.","path": [
    "_entities","createdAt"
  ],"extensions": {
    "code": "INTERNAL_SERVER_ERROR","serviceName": "XXX","query": "query($representations:[_Any!]!){_entities(representations:$representations){...on Transaction{createdAt}}}","variables": {
      "representations": [
        {
          "__typename": "Transaction","id": "29bf75e5-b79e-4a7d-a021-84a8b5662aa8"
        },{
          "__typename": "Transaction","id": "616f3f8a-3c81-4d2e-bce0-03d031a15062"
        }
      ]
    },//etc

为什么variables.representations缺少createdAt值？当我直接查询DynamoDB时，我可以看到所有这两个项目的createdAt值。

我的模式如下：

extend type Transaction @key(fields: "id") {
  id: ID! @external
}

type Query {
  getMyTransactions: [Transaction!]!
}

另一个架构的交易类型为：

type Transaction @key(fields: "id") {
  id: ID!
  createdAt: String!
}