使用AppSync JS解析器查询DynamoDB返回null问题排查求助

自定义AppSync JS解析器查询DynamoDB时始终返回null，但CloudWatch日志明确显示已经成功获取到数据，请求帮忙定位问题。

AppSync 查询 #

JS 解析器代码 #

import { util } from '@aws-appsync/utils';
import * as ddb from "@aws-appsync/utils/dynamodb";

export function request(ctx) {
  const { PK, limit = 20, nextToxen} = ctx.args;
  return ddb.query({
      query: {PK: {eq:  "SUE#" + PK} },
      limit,
      nextToxen,
  });
}

export function response(ctx) {
    return ctx.result;
}

CloudWatch 日志 #

{
    "logType": "AfterResponseFunctionEvaluation",
    "path": [
        "querySuE"
    ],
    "fieldName": "querySuE",
    "resolverArn": "arn:aws:appsyncxxxxxx",
    "requestId": "xxxxxxxxx",
    "context": {
        "arguments": {
            "PK": "zzzzz"
        },
        "result": {
            "items": [
                {
                    "createdAt": "2024-06-09T20:40:30.940Z",
                    "SK": "SUE#01HZZBPBEWXBCSMZMKG8MHF16Q",
                    "PK": "SUE#zzzzz",
                    "_TYPE": "SUE",
                    "title": "ichbintitel"
                },
                {
                    "createdAt": "2024-06-09T20:40:31.468Z",
                    "SK": "SUE#01HZZBPBZC6Q4WW003GQR1HXWC",
                    "PK": "SUE#zzzzz",
                    "_TYPE": "SUE",
                    "title": "ichbintitel"
                }
            ],
            "scannedCount": 2
        },
        "prev": {
            "result": {
                "items": [
                    {
                        "createdAt": "2024-06-09T20:40:30.940Z",
                        "SK": "SUE#01HZZBPBEWXBCSMZMKG8MHF16Q",
                        "PK": "SUE#zzzzz",
                        "_TYPE": "SUE",
                        "title": "ichbintitel"
                    },
                    {
                        "createdAt": "2024-06-09T20:40:31.468Z",
                        "SK": "SUE#01HZZBPBZC6Q4WW003GQR1HXWC",
                        "PK": "SUE#zzzzz",
                        "_TYPE": "SUE",
                        "title": "ichbintitel"
                    }
                ],
                "scannedCount": 2
            }
        },
        "stash": {},
        "outErrors": []
    },
    "fieldInError": false,
    "evaluationResult": {
        "items": [
            {
                "createdAt": "2024-06-09T20:40:30.940Z",
                "SK": "SUE#01HZZBPBEWXBCSMZMKG8MHF16Q",
                "PK": "SUE#zzzzz",
                "_TYPE": "SUE",
                "title": "ichbintitel"
            },
            {
                "createdAt": "2024-06-09T20:40:31.468Z",
                "SK": "SUE#01HZZBPBZC6Q4WW003GQR1HXWC",
                "PK": "SUE#zzzzz",
                "_TYPE": "SUE",
                "title": "ichbintitel"
            }
        ],
        "scannedCount": 2
    },
    "errors": [],
    "parentType": "Query",
    "graphQLAPIId": "xxxxxxxxx"
}

问题定位与解决建议 #

从CloudWatch日志可以确认，解析器已经成功从DynamoDB获取到数据（evaluationResult包含完整的items列表），返回null的核心原因几乎都是GraphQL Schema定义与解析器返回的数据结构不匹配，按以下步骤排查：

1. 核对GraphQL Schema的返回类型定义

   • 确保Query类型中的querySuE字段返回类型，和解析器返回的{ items: [...], scannedCount: number }结构一致。比如是否定义了对应的嵌套类型：

     type SUEItem {
         createdAt: String
         SK: String
         PK: String
         _TYPE: String
         title: String
     }
     
     type SUEQueryResult {
         items: [SUEItem]
         scannedCount: Int
     }
     
     type Query {
         querySuE(PK: String!, limit: Int, nextToken: String): SUEQueryResult
     }
     

   • 如果Schema里querySuE被定义为直接返回[SUEItem]（列表类型），而解析器返回的是包含items的对象，结构不匹配就会导致客户端拿到null。

2. 调整解析器返回值结构

   • 如果Schema期望直接返回数据列表，修改response函数，返回ctx.result.items而非整个ctx.result：

     export function response(ctx) {
         return ctx.result.items;
     }
     

3. 修正参数拼写错误

   • 解析器中nextToxen是拼写错误，正确应为nextToken，虽然本次请求未传该参数，但后续分页功能会受影响，建议修正。

内容的提问来源于stack exchange，提问作者Sami