AppSync Resolver only works when I hard code the input. context.arguments does not work - amazon-dynamodb

Question

Edit for clarity: There are no error messages, it simply returns an empty list if the input string is from the context.arguments, suggesting that it simply isn't getting the input variable out on the query tester (setting it up incorrectly brings up that famous typing error of course). I've also made this into a pipeline with the exact same result. Looking around, people suggest making an intermediate object, but surely I'm just getting my input variables out wrong somehow.
I'm working on a project in AWS Appsync using DynamoDB and I've run into a problem with the context.arguments input.
Basically the code all works if I hardcode the string for the book id into the query (full context to follow), but if I use the context.arguments, it simply refuses to work properly, returning an empty array for the "spines".
I have the following types in my schema:
type Book {
id: ID!
title: String
spines: [Spine]
}
type Spine {
id: ID!
name: String
bookId: ID!
}
I use the following query:
type Query {
getBook(id: ID!): Book
query getBook($bookId: ID!){
getBook(id: $bookId){
title
id
spines {
name
bookId
}
}
}
With the following input (assume this is a relevant guid):
{
"bookId": "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
}
And this resolver for the spines object:
{
"version" : "2017-02-28",
"operation" : "Query",
"index" : "bookId-index",
"query" : {
"expression": "#bookId = :bookId",
"expressionNames" : {
"#bookId" : "bookId"
},
"expressionValues" : {
":bookId" : { "S" : "${context.arguments.id}" }
}
}
}
}
I made sure my data set contained false positives too (spines for other books) so that I know when my query brings back the correct data.
This works if I hardcode a guid as string instead of using context.arguments, and gets exactly what I'm looking for for each book guid.
For example, replacing the expression values with this works perfectly:
"expressionValues" : {
":bookId" : { "S" : "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa" }
}
Why does "${context.arguments.id}" not get the input variable here the same way as it seems to in other queries?

Answer 1

Thanks to #IonutTrestian for pointing me in the right direction.
$ctx.args was empty, but I decided to go up the chain to see what was in the entire context, so $util.error($util.toJson($ctx)).
The json object I found included a little object called "Source", which contained the query return for the Book object.
Long story short, $ctx.source.id when applied to my query worked a charm.
I also know a bit more about debugging DynamoDB resolvers in case I encounter problems like this in future. Thank you so much!