Create JSON Object with Velocity Template Language

Question

I am working with the Apache Velocity Template Language (VTL) in AWS AppSync. In my request template I query an undefined amount of items with different elements. What I want to do in the response template is to transform the result into a JSON Object which I need later on for my BatchDeleteItem operation. This is how I solved it currently:

#set($deleteObject='{"Id" : { "S": "nodelete" },"Sk" : { "S": "nodelete" }}')
#set($replaceDeleteObject ="")
#set($separator="")

#foreach( $item in $ctx.result.items )
  #set($replaceDeleteObject = $replaceDeleteObject + $separator + '{"Id" : { "S": "' + $item.Id + '" },"Sk" : { "S": "' + $item.Sk + '" }}')
  #set($separator = ",")
  #set($deleteObject = $replaceDeleteObject)
#end

$util.qr($ctx.stash.put("deleteObject", $deleteObject))

Later on I can access my deleteObject and it works properly.

My question however is, if it is possible to directly create some kind of JSON Object in vtl in which I can append my values instead of creating this string in the form of a JSON Object?

score 3 · Accepted Answer · answered Jun 29 '19 at 06:37

You can follow this example from the AppSync Batch docs:

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

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

So for your case it should look like this:

#set ($batchDeleteArray = [])
#foreach( $item in $ctx.result.items )
  #set ($entry = {
    "Id": $util.dynamodb.toString($item.Id),
    "Sk": $util.dynamodb.toString($item.Sk)
  })

  $util.qr($batchDeleteArray.add($entry))
#end

{
    "version" : "2018-05-29",
    "operation" : "BatchDeleteItem",
    "tables" : {
        "TableName": {
          "keys": $util.toJson($batchDeleteArray)
        }
    }
}

score 1 · Answer 2 · answered Mar 18 '19 at 05:41

1

You can create a map/dictionary which is close to JSON object that you need. There is a programming guide here which should have the information you need, but essentially what you want is something like this.

#set($deleteObject={
     "Id" : $util.dynamodb.toString("nodelete"),
     "Sk" : $util.dynamodb.toString("nodelete")
})

answered Mar 18 '19 at 05:41

Karthik

934
9
21

Unfortunately I have multiple items which would append to an object array. That is not possible with the map/dictonary – benra May 22 '19 at 09:30

Create JSON Object with Velocity Template Language

2 Answers2