AWS AppSync & DynamoDB Single Table Design: PutItem based on another item's field value

Question

I'm new to AWS AppSync and I have adopted the single table pattern in DynamoDB. Now I am trying to create an item based on a particular field value in the existing item in the same table. For example, I have a table called transaction which holds 2 types of records.

1. Request
2. Response

As you can see the above table, I can insert (PutItem) multiple responses for a particular request. Before I insert a new response, I need to validate whether the request (RequestID) is already exists. Is there any way to do via conditional expression in the resolver? Below is my current request resolver code which is not working as expected.

#set( $Id = $util.autoId() )
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
    "PK": $util.dynamodb.toDynamoDBJson("USER#$ctx.args.input.UserId"),
    "SK": $util.dynamodb.toDynamoDBJson("RESPONSE#$Id"),
},
"attributeValues" : $util.dynamodb.toMapValuesJson($ctx.args.input),
 "condition": {
        "expression": "SK = :SK",
        "expressionValues" : {
                   ":SK" : {
                       "S" : "REQUEST#${ctx.args.input.RequestId}"
                   }
     }
  }

}

Answer 1

You could do this in one request mapping template by using DynamoDB transactions, see (https://docs.aws.amazon.com/appsync/latest/devguide/tutorial-dynamodb-transact.html).

In one of the "transactWriteItems", you update an arbitrary value (or perhaps, number of responses) in the request item with a condition that checks if the request item exists with that requestId in the SK. If the conditions succeeds, the request item is updated.

Also make sure you have the response item in your "transactWriteItems" so that the response item is also written if the request condition passes.

Answer 2

You won't be able to do a conditional PutItem based on another entry no. In this case you'd want to use pipeline resolvers. In your first function you'd fetch the request item and in the second you can then do your PutItem condition - the previous GetItem result will be available as $ctx.prev.result.