AWS AppSync resolver template to dynamically put stuff in a DynamoDB

update.vtl

{
    "version" : "2017-02-28",
    "operation" : "UpdateItem",
    "key" : {
        "id" : { "S" : "${ctx.args.id}" }
    },

    #set( $expNames  = {} )
    #set( $expValues = {} )
    #set( $expSet = {} )

    #foreach( $entry in $ctx.args.entrySet() )
        #if( $entry.key != "id")
            $!{expSet.put("#${entry.key}", ":${entry.key}")}
            $!{expNames.put("#${entry.key}", "$entry.key")}
            $!{expValues.put(":${entry.key}", $util.dynamodb.toDynamoDB($entry.value))}
        #end
    #end
    #set( $expression = "SET" )
    #foreach( $entry in $expSet.entrySet() )
        #set( $expression = "${expression} ${entry.key} = ${entry.value}" )
        #if ( $foreach.hasNext )
            #set( $expression = "${expression}," )
        #end
    #end

    "update" : {
        "expression" : "${expression}",
        "expressionNames" : $utils.toJson($expNames),
        "expressionValues" : $utils.toJson($expValues),
    }
}