AWS Appsync Multi-table BatchPutItem Returns Null - amazon-web-services

Question

I've been following the tutorial for batch operations:
https://docs.aws.amazon.com/appsync/latest/devguide/tutorial-dynamodb-batch.html#multi-table-batch
When I tried to do a multi-table batch, I get a null response even though the objects are successfully submitted to dynamodb.
This is my mutation:
mutation sendReadings {
recordReadings(
tempReadings: [
{sensorId: 1, value: 85.5, timestamp: "2018-02-01T17:21:05.000+08:00"},
{sensorId: 2, value: 85.7, timestamp: "2018-02-01T17:21:06.000+08:00"},
{sensorId: 3, value: 85.8, timestamp: "2018-02-01T17:21:07.000+08:00"},
{sensorId: 4, value: 84.2, timestamp: "2018-02-01T17:21:08.000+08:00"},
{sensorId: 5, value: 81.5, timestamp: "2018-02-01T17:21:09.000+08:00"}
]
locReadings: [
{sensorId: 1, lat: 47.615063, long: -122.333551, timestamp: "2018-02-01T17:21:05.000+08:00"},
{sensorId: 2, lat: 47.615163, long: -122.333552, timestamp: "2018-02-01T17:21:06.000+08:00"}
{sensorId: 3, lat: 47.615263, long: -122.333553, timestamp: "2018-02-01T17:21:07.000+08:00"}
{sensorId: 4, lat: 47.615363, long: -122.333554, timestamp: "2018-02-01T17:21:08.000+08:00"}
{sensorId: 5, lat: 47.615463, long: -122.333555, timestamp: "2018-02-01T17:21:09.000+08:00"}
]) {
locationReadings {
sensorId
timestamp
lat
long
}
temperatureReadings {
sensorId
timestamp
value
}
}
}
The problem is that the response returns null:
{
"data": {
"recordReadings": {
"locationReadings": null,
"temperatureReadings": null
}
}
}
Request Mapping Template:
## Convert tempReadings arguments to DynamoDB objects
#set($tempReadings = [])
#foreach($reading in ${ctx.args.tempReadings})
$util.qr($tempReadings.add($util.dynamodb.toMapValues($reading)))
#end
## Convert locReadings arguments to DynamoDB objects
#set($locReadings = [])
#foreach($reading in ${ctx.args.locReadings})
$util.qr($locReadings.add($util.dynamodb.toMapValues($reading)))
#end
{
"version" : "2018-05-29",
"operation" : "BatchPutItem",
"tables" : {
"LocationReadingTable": $utils.toJson($locReadings),
"TemperatureReadingTable": $utils.toJson($tempReadings)
}
}
Response Mapping Template:
## If there was an error with the invocation
## there might have been partial results
#if($ctx.error)
## Append a GraphQL error for that field in the GraphQL response
$utils.appendError($ctx.error.message, $ctx.error.message)
#end
## Also returns data for the field in the GraphQL response
$utils.toJson($context.result.data)
The code is pretty much identical to the tutorial. I checked the logs and the response is being sent but it's not being captured so either my code is off or there is a bug on AWS's side. Can anyone reproduce this or is able to get a response when doing a multi-table BatchPutItem?

Answer 1

Thank you for a very detailed question, it certainly helped me debug your issue. So, in order to fix your issue, you need to tweak your response mapping template. I was able to reproduce the issue with null response on my end. The reason being that the shape of the multi-table batch put response is of this format:
"result": {
"data": {
"TemperatureReadingTable": [
{
"value": 85.5,
"sensorId": "1",
"timestamp": "2018-02-01T17:21:05.000+08:00"
},
{
"value": 85.7,
"sensorId": "2",
"timestamp": "2018-02-01T17:21:06.000+08:00"
},
{
"value": 85.8,
"sensorId": "3",
"timestamp": "2018-02-01T17:21:07.000+08:00"
},
{
"value": 84.2,
"sensorId": "4",
"timestamp": "2018-02-01T17:21:08.000+08:00"
},
{
"value": 81.5,
"sensorId": "5",
"timestamp": "2018-02-01T17:21:09.000+08:00"
}
],
"LocationReadingTable": [
{
"lat": 47.615063,
"long": -122.333551,
"sensorId": "1",
"timestamp": "2018-02-01T17:21:05.000+08:00"
},
{
"lat": 47.615163,
"long": -122.333552,
"sensorId": "2",
"timestamp": "2018-02-01T17:21:06.000+08:00"
},
{
"lat": 47.615263,
"long": -122.333553,
"sensorId": "3",
"timestamp": "2018-02-01T17:21:07.000+08:00"
},
{
"lat": 47.615363,
"long": -122.333554,
"sensorId": "4",
"timestamp": "2018-02-01T17:21:08.000+08:00"
},
{
"lat": 47.615463,
"long": -122.333555,
"sensorId": "5",
"timestamp": "2018-02-01T17:21:09.000+08:00"
}
]
},
So, the $context.result.data will return you an object with TemperatureReadingTable and LocationReadingTable as its fields. However, you are applying $util.toJson to resolve the type RecordResult, which has temperatureReadings and locationReadings as child fields.
Please update your Response Mapping template with the following definition, and it will work for you:
#if($ctx.error)
## Append a GraphQL error for that field in the GraphQL response
$utils.appendError($ctx.error.message, $ctx.error.message)
#end
{
"temperatureReadings": $util.toJson(${ctx.result.data.TemperatureReadingTable}),
"locationReadings": $util.toJson(${ctx.result.data.LocationReadingTable})
}
Moreover, I'd encourage you enable CloudWatch logs from the Settings page of the Console, with ALL as the option. This will log Request/Response headers, the resolved Request/Response templates, Tracing information, etc. So it'll help you identify issues like these quickly by looking at the content of the request/response templates.
Thank you for bringing this issue, we will update the documentation, if it does not mention this.