AWS Appsync Batch Resolver

Question

Struggling with this for sometime now, and applogies I changed the query name for the question to getDeviceReadings, I have been using getAllUserDevices (sorry for any confusion)

type Device {
   id: String
   device: String!
}

type Reading {
   device: String
   time: Int
}

type PaginatedDevices {
   devices: [Device]
   readings: [Reading]
   nextToken: String
}

type Query {
   getDevicesReadings(nextToken: String, count: Int): PaginatedDevices
}

Then I have a resolver on the query getDevicesReadings which works fine and returns all the devices a user has so far so good

{
"version": "2017-02-28",
"operation": "Query",
"query" : {
  "expression": "id = :id",
    "expressionValues" : {
      ":id" : { "S" : "${context.identity.username}" }
    }
}
#if( ${context.arguments.count} )
    ,"limit": ${context.arguments.count}
#end
#if( ${context.arguments.nextToken} )
    ,"nextToken": "${context.arguments.nextToken}"
#end
}

now I want to return all the readings that devices has based on the source result so I have a resolver on getDevicesReadings/readings

#set($ids = [])
#foreach($id in ${ctx.source.devices})
  #set($map = {})
  $util.qr($map.put("device", $util.dynamodb.toString($id.device)))
  $util.qr($ids.add($map))
#end

{
"version" : "2018-05-29",
"operation" : "BatchGetItem",
 "tables" : {
    "readings": {
        "keys": $util.toJson($ids),
        "consistentRead": true
    }
  }
}

With a response mapping like so ..

$utils.toJson($context.result.data.readings)

I run a query

query getShit{
  getDevicesReadings{
    devices{
      device
     }
    readings{
      device
      time
    }
  }
}

this returns the following results

{
  "data": {
    "getAllUserDevices": {
     "devices": [
       {
         "device": "123"
       },
       {
         "device": "a935eeb8-a0d0-11e8-a020-7c67a28eda41"
       }
     ],
     "readings": [
       null,
       null
     ]
   }
 }
}

As you can see on the image the primary partition key is device on the readings table I look at the logs and I have the following

Sorry if you cant read the log it basically says that there are unprocessedKeys

and the following error message

"message": "The provided key element does not match the schema (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ValidationException; Request ID: 0H21LJE234CH1GO7A705VNQTJVVV4KQNSO5AEMVJF66Q9ASUAAJG)",

I'm guessing some how my mapping isn't quite correct and I'm passing in readings as my keys ?

Any help greatly appreciated

Answer 1

You're correct, the request mapping template you provided doesn't match the primary key on the readings table. A BatchGetItem expects keys to be primary keys, however you are only passing the hash key.

For the BatchGetItem call to succeed you must pass both hash and sort key , so in this case, both device and time attributes.

Maybe a Query on the readings table would be more appropriate?

Answer 2

No, you can absolutely use batch resolvers when you have a primary sort key. The error in your example is that you were not providing the primary sort key to the resolver.

This code needs to provide a "time" as well a "device" because you need both to fully specify the primary key.

#set($ids = [])
#foreach($id in ${ctx.source.devices})
  #set($map = {})
  $util.qr($map.put("device", $util.dynamodb.toString($id.device)))
  $util.qr($ids.add($map))
#end

You should have something like it:

#set($ids = [])
#foreach($id in ${ctx.source.devices})
  #set($map = {})
  # The tables primary key is made up of "device" AND "time"
  $util.qr($map.put("device", $util.dynamodb.toString($id.device)))
  $util.qr($map.put("time", $util.dynamodb.toString($id.time)))
  $util.qr($ids.add($map))
#end

If you want to get many records that share the same "device" value but that have different "time" values, you need to use a DynamoDB Query operation, not a batch get.

Answer 3

So you can't have a batch resolver when you have primary sort key ?!

So the answer was to create a lambda function and tack that on as my resolver

import boto3
from boto3.dynamodb.conditions import Key

def lambda_handler(event, context):

   list = []
   for device in event['source']['devices'] :
       dynamodb = boto3.resource('dynamodb')
       readings  = dynamodb.Table('readings')
       response = readings.query(
           KeyConditionExpression=Key('device').eq(device['device'])
       )
       items = response['Items']
       list.extend(items)
   return list