GraphQL field resolver needs contextual information - api

Question

Maybe my terminology is not accurate. I'm using AWS AppSync. My schema:
type Book {
title: String
author: Author
}
type Author {
name: String
}
type Query {
getBook(title:String!): Book
}
The resolver for getBook returns an object shaped as:
{
title: <string>
authorId: <number>
}
Where authorId is always returned.
What I'd like to do is specify a resolver for the field Book.author that will receive authorId and fetch that object from its own data store. Is this possible?
If what I'm trying to do is not possible, what is the proper way of doing this, where one data store is a table with two columns - { title, authorId }, and a separate store has a table with a list of authors, where the primary key is a column authorId. Since these are two different services, I can't just join the two like a SQL query.

Answer 1

As long as authorId is returned from the getBook resolver, it will be accessible via $ctx.source.authorId when resolving Book.author.
I reproduced your API with local resolvers using your schema:
Query.getBook request mapping template:
{
"version": "2018-05-29",
"payload": {
"title": "$context.arguments.title",
"authorId": "2" ## returned in addition to other fields. It will be used by Book.author resolver.
}
}
Query.getBook response mapping template:
$util.toJson($context.result)
Book.author request mapping template:
{
"version": "2018-05-29",
"payload": {
"name": "author name with authorId: $context.source.authorId"
}
}
Book.author response mapping template:
$util.toJson($context.result)
The following query:
query {
getBook(title:"AWS AppSync") {
title
author {
name
}
}
}
will yield the results:
{
"data": {
"getBook": {
"title": "AWS AppSync",
"author": {
"name": "author name with authorId: 2"
}
}
}
}

Answer 2

You might need to have bookID as parent's ID inside Author:
type Author {
# parent's id
bookID: ID!
# author id
id: ID!
name: String!
}
type Book {
id: ID!
title: String!
author: Author!
}
When Create Resource, just make:
- Book.id as primary key of BookTable
- Author.bookID as primary key and Author.id as sort key of
AuthorTable
You also need to attach resolver for Book.author using $ctx.source.id
After you attach Book.author resolver, you are good to go. You can get result something like below:
getBook(title: "xx") {
id
title
author {
id
name
}
}