how to handle nested lists in AWS APIG Mapping Template in VTL - amazon-web-services

Question

(Here's my Model scheme:
{
"$schema": "http://json-schema.org/draft-04/schema#",
"title": "QuestionsModel",
"type": "array",
"items": {
"type": "object",
"properties": {
"section_name": { "type": "string" },
"options" : {
"type" : "array",
"items" : {
"type" : "array",
"items" : {
"type" : "string"
}
}
}
}
Here's the Mapping template:
#set($inputRoot = $input.path('$'))
[
#foreach($question in $inputRoot) {
"section_name" : "$question.section_name.S",
"options" : [
#foreach($items in $question.options.L) {
[
#foreach($item in $items.L) {
"$item.S"
}#if($foreach.hasNext),#end
#end
]
}#if($foreach.hasNext),#end
#end
]
}#if($foreach.hasNext),#end
#end
]
Although this syntax correctly maps the data it results in "options" being an empty array.
Without the "options" specified then my iOS app receives valid JSON. But when I try various syntaxes for "options" then I either get invalid JSON or an "Internal Service Error" and CloudWatch isn't much better offering Unable to transform response.
The options valid is populated with this content: {L=[{"L":[{"S":"1"},{"S":"Dr"}]},{"L":[{"S":"2"},{"S":"Mr"}]},{"L":[{"S":"3"},{"S":"Ms"}]},{"L":[{"S":"4"},{"S":"Mrs"}]},{"L":[{"S":"5"},{"S":"Prof."}]}]} which is provided by a Lambda function.
I can only conclude, at this point, that API Gateway VTL doesn't support nested arrays.

Answer 1

AWS iOS SDK for Modelling doesn't support array of arrays.
You have to define a dictionary in between any nested arrays.
So instead of array/object/array/array you slip in an extra "awshack" object: array/object/array/awshack-object/array
{
"$schema": "http://json-schema.org/draft-04/schema#",
"title": "QuestionsModel",
"type": "array",
"items": {
"type": "object",
"properties": {
"section_name": { "type": "string" },
"options" : { "type" : "array",
"items" : {
"type" : "object",
"properties" : {
"awshack" : {
"type" : "array",
"items" : { "type" : "string" }
}
}
}
}
}
}
}
In the mapping template the "awshack" is slipped in outside the innermost loop.
#foreach($items in $question.options.L)
{"awshack" :
[#foreach($item in $items.L)
"$item.S"#if($foreach.hasNext),#end
#end
#if($foreach.hasNext),#end
]}#if($foreach.hasNext),#end
#end
Amazon confirms this limitation.