How to resolve parent to child relationship with AppSync - amazon-web-services

Question

I have schema looking like below
type Post {
id: ID!
creator: String!
createdAt: String!
like: Int!
dislike: Int!
frozen: Boolean!
revisions:[PostRevision!]
}
type PostRevision {
id: ID!
post: Post!
content: String!
author: String!
createdAt: String!
}
type Mutation {
createPost(postInput: CreatePostInput!): Post
}
I would like to be able to batch insert Post and PostRevision at the same time when i run createPost mutation; however, VTL is giving me a much of hard time.
I have tried below
## Variable Declarations
#set($postId = $util.autoId())
#set($postList = [])
#set($postRevisionList = [])
#set($post = {})
#set($revision = {})
## Initialize Post object
$util.qr($post.put("creator", $ctx.args.postInput.author))
$util.qr($post.put("id", $postId))
$util.qr($post.put("createdAt", $util.time.nowEpochMilliSeconds()))
$util.qr($post.put("like", 0))
$util.qr($post.put("dislike", 0))
$util.qr($post.put("frozen", false))
## Initialize PostRevision object
$util.qr($revision.put("id", $util.autoId()))
$util.qr($revision.put("author", $ctx.args.postInput.author))
$util.qr($revision.put("post", $postId))
$util.qr($revision.put("content", $ctx.args.postInput.content))
$util.qr($revision.put("createdAt", $util.time.nowEpochMilliSeconds()))
## Listify objects
$postList.add($post)
$postRevisionList.add($revision)
{
"version" : "2018-05-29",
"operation" : "BatchPutItem",
"tables" : {
"WHISPR_DEV_PostTable": $util.toJson($postList),
"WHISPR_DEV_PostRevisionTable": $util.toJson($postRevisionList)
}
}
So basically I am reconstructing the document in the resolver of createPost so that I can add Post then also add ID of the post to postReivision However when I run below code
mutation insertPost{
createPost(postInput:{
creator:"name"
content:"value"
}){
id
}
}
I get following error
{
"data": {
"createPost": null
},
"errors": [
{
"path": [
"createPost"
],
"data": null,
"errorType": "MappingTemplate",
"errorInfo": null,
"locations": [
{
"line": 2,
"column": 3,
"sourceName": null
}
],
"message": "Expected JSON object but got BOOLEAN instead."
}
]
}
What am I doing wrong?
I know it would be easier to resolve with lambda function but I do not want to double up the cost for no reason. Any help would be greatly appreciated. Thanks!

Answer 1

If anyone still needs the answer for this (this question is still the #1 google hit for the mentioned error message):
The problem is the return value of the add() method, which returns a boolean value.
To fix this, just wrap the add() methods into $util.qr, as you are already doing for the put() methods:
$util.qr(($postList.add($post))
$util.qr(($postRevisionList.add($revision))

Answer 2

It looks like you are missing a call to $util.dynamodb.toDynamoDBJson which is causing AppSync to try to put plain JSON objects into DynamoDB when DynamoDB requires a DynamoDB specific input structure where each attribute instead of being a plain string like "hello world!" is an object { "S": "hello world!" }. The $util.dynamodb.toDynamoDBJson helper handles this for you for convenience. Can you please try adding the toDynamoDBJson() to these lines:
## Listify objects
$postList.add($util.dynamodb.toDynamoDBJson($post))
$postRevisionList.add($util.dynamodb.toDynamoDBJson($revision))
Hope this helps :)