AWS Appsync Batch Resolver - amazon-web-services

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

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 2

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 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