Populate nested fields in aws appsync vtl resolvers

Post.author.req.vtl

## Problem: How to populate all authors in a post in Appsync using Vtl resovlers?!

## Post(Parent Query) Structure: 
## {
##    id: ID!
##    authorID: ID!
##    author: User
##    title: String!
## }

## Solution: 
## We can populate author details in the 'author' field. 
## We'll read authorID from '$ctx.source.author' and pass it as primary_key to the Nested Query. 

## Steps: 
## 1. Attach a resolver to author field. 
## 2. Paste in the following code in the resolver. 

{
    "version": "2017-02-28",
    "operation": "GetItem",
    "key": {
        "sub": $util.dynamodb.toDynamoDBJson($ctx.source.author),
    }
}

## Conclusion: Nested fields can be resolved by attaching a resolver against that field.