VTL: Add a new property to an maps inside an array (AWS Appsync, DynamoDB) - amazon-dynamodb

Question

I'm new to VTL and AWS appsync and try to get my head around how things are working.
The maps that are representing Steps in a List should have the property id with a UUID before there are stored in DynamoDB. To accomplish this I tried to iterate over the array and access the put method on the map like in the example below.
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key": {
"id" : $util.dynamodb.toDynamoDBJson($util.autoId())
},
#set($input = $util.dynamodb.toMapValues($ctx.args.input))
#set($input.createdAt = $util.dynamodb.toDynamoDB($util.time.nowISO8601()))
#foreach($step in $input.steps)
$step.put('id', $util.dynamodb.toDynamoDBJson($util.autoId())))
#end
"attributeValues": $util.toJson($input)
}
my second try:
#foreach($step in $input.steps)
#set($step.id = $util.dynamodb.toDynamoDBJson($util.autoId()))
#end
but still, for no reason, my maps do not have the property id.
Is there maybe the problem what the foreach loop is giving me just a copy of the map I try to modify and not the original object?
Thank you for your time!
Hopefully, my question will serve all newbies to VTL and appsync

Answer 1

They do, it's just that the toMapValues utility method is returning you DynamoDB types.
So if "input.steps" is supposed to be a list what you're gonna get in there is an object like {"L": [ ... ]}
Try this:
#foreach($step in $input.steps.L)
$step.put('id', $util.dynamodb.toDynamoDBJson($util.autoId())))
#end