How to manipulate strings and array in Apache Velocity request mapping template in AWS appSync

Question

This is my first time working with VTL, so please correct my code if there is something very silly in it.

What I want to achieve

{
    "cities": ["jaipur", "mumbai", "delhi", "sheros", "jalandhar", "bengaluru"]
}

Graphql schema:

type Query{
  getCitiesForGroups: String
}

Default response template:

#if($ctx.error)
    $utils.error($ctx.error.message, $ctx.error.type)
#end
    $utils.toJson($utils.rds.toJsonObject($ctx.result)[0])

Using the default response template, the result I was getting

{ "data": { "getCitiesForGroups": [ "{groupcity=jaipur}", "{groupcity=mumbai}", "{groupcity=delhi}", "{groupcity=sheros}", "{groupcity=jalandhar}", "{groupcity=bengaluru}" ] } }

My request template

{
    "version": "2018-05-29",
    "statements": [
        "select DISTINCT LOWER(city) as city from public.Groups"
    ]
}

To get the desired output, I changed the response template as I wanted to loop over the response I got from the DB, and remove the {city=} from the string by using the substring method given in AWS resolver mapping docs and this is where I am facing the problem.

My response template

#if($ctx.error)
    $utils.error($ctx.error.message, $ctx.error.type)
#end
#set ($rawListOfCities = $utils.rds.toJsonObject($ctx.result)[0])
#set ($sanitisedListOfCities = [])
#foreach( $city in $rawListOfCities )
    #set ($equalToIndex = $city.indexOf("="))
    #set ($equalToIndex = $equalToIndex + 1)
    #set ($curlyLastIndex = $city.lastIndexOf("}"))
    #set ($tempCity = $city.substring($equalToIndex, $curlyLastIndex))
    ## #set ($tempCity = $city)
    $util.qr($sanitisedListOfCities.add($tempCity))
#end
$util.toJson($sanitisedListOfCities)

The response that I am getting:

{
  "data": {
    "getCitiesForGroups": "[null, null, null, null, null, null]"
  }
}

However, when I use the line #set ($tempCity = $city) and comment out the line above it, I get the following response:

{
  "data": {
    "getCitiesForGroups": "[{city=jaipur}, {city=mumbai}, {city=delhi}, {city=sheros}, {city=jalandhar}, {city=bengaluru}]"
  }
}

Which means $city has the value {city=jaipur}, so which I want to sanitize and add it to ($sanitisedListOfCities) and return it as the response.

But I am getting null as the result substring method.

So how can I sanitize the response from DB and return it?

score 4 · Accepted Answer · answered May 14 '20 at 18:32

I had a similar issue and this is how I solved it.

First, your Graphql schema should look like,

type Query {
    getCitiesForGroups: cityList
}

type cityList {
    cities: [String]
}

Your request mapping template,

{
    "version": "2018-05-29",
    "statements": [
        "select DISTINCT LOWER(city) as city from public.Groups"
    ]
}

And finally your response mapping template

#set($cityList = [])
#set($resMap = {})
#if($ctx.error)
    $utils.error($ctx.error.message, $ctx.error.type)
#end

#foreach($item in $utils.rds.toJsonObject($ctx.result)[0])
    $util.qr($cityList.add($item.city))
#end
$util.qr($resMap.put("cities", $cityList))

#return($resMap)

Expected response(complete)

{
  "data": {
    "getCitiesForGroups": {
      "cities": [
        "jaipur",
        "mumbai",
        "delhi",
        "sheros",
        "jalandhar",
        "bengaluru"
      ]
    }
  }
}

I hope this helps you.

Your solution works like a charm. Thank you so much for helping out. Can you please point me to some resources so I can understand this all much better? Thank you once again. — Sajal, May 14 '20 at 19:11
Glad to help you. I usually refer to AWS Developer Guide, Stackoverflow and sometimes AWS Discussion forum. Other than that, Adrian Hall and Nader Dabit have some amazing tutorials and guides on AppSync; you should also check them out! — Myz, May 15 '20 at 03:12

How to manipulate strings and array in Apache Velocity request mapping template in AWS appSync

1 Answers1