How to properly format data with AppSync and DynamoDB when Lambda is in between

Question

Receiving data with AppSync directly from DynamoDB seems working for my case, but when I try to put a lambda function in between, I receive errors that says "Can't resolve value (/issueNewMasterCard/masterCards) : type mismatch error, expected type LIST"

Looking to the AppSync cloudwatch response mapping output, I get this:

 "context": {
        "arguments": {
            "userId": "18e946df-d3de-49a8-98b3-8b6d74dfd652"
        },
        "result": {
            "Item": {
                "masterCards": {
                    "L": [
                        {
                            "M": {
                                "cardId": {
                                    "S": "95d67f80-b486-11e8-ba85-c3623f6847af"
                            },
                                "cardImage": {
                                    "S": "https://s3.eu-central-1.amazonaws.com/logo.png"
                                },
                                "cardWallet": {
                                    "S": "0xFDB17d12057b6Fe8c8c434653456435634565"
                                },...............

here is how I configured my response mapping template:

$utils.toJson($context.result.Item)

I'm doing this mutation:

mutation IssueNewMasterCard {
  issueNewMasterCard(userId:"18e946df-d3de-49a8-98b3-8b6d74dfd652"){
    masterCards {
      cardId
    }
  }
}

and this is my schema :

type User {
    userId: ID!
    masterCards: [MasterCard]
}

type MasterCard {
    cardId: String
}

type Mutation {
    issueNewMasterCard(userId: ID!): User
}

The Lambda function:

exports.handler = (event, context, callback) => {

    const userId = event.arguments.userId;

         const userParam = {
              Key: {
                 "userId":{S:userId}
              },
              TableName:"FidelityCardsUsers"
             }

      dynamoDB.getItem(userParam, function(err, data) {

        if (err) {
            console.log('error from DynamDB: ',err)
            callback(err);
        } else {
            console.log('mastercards: ',JSON.stringify(data));
            callback(null,data)
        }

 })

Answer 1

I think the problem is that the getItem you use when you use the DynamoDB datasource is not the same as the the DynamoDB.getItem function in the aws-sdk.

Specifically it seems like the datasource version returns an already marshalled response (that is, instead of something: { L: [ list of things ] } it just returns something: [ list of things]).

This is important, because it means that $utils.toJson($context.result.Item) in your current setup is returning { masterCards: { L: [ ... which is why you are seeing the type error- masterCards in this case is an object with a key L, rather than an array/list.

To solve this in the resolver, you can use the $util.dynamodb.toDynamoDBJson(Object) macro (https://docs.aws.amazon.com/appsync/latest/devguide/resolver-util-reference.html#dynamodb-helpers-in-util-dynamodb). i.e. your resolver should be:

$util.dynamodb.toDynamoDBJson($context.result.Item)

Alternatively you might want to look at the AWS.DynamoDB.DocumentClient class (https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB/DocumentClient.html). This includes versions of getItem, etc. that automatically marshal and unmarshall the proprietary DynamoDB typing back into native JSON. (Frankly I find this much nicer to work with and use it all the time).

In that case you can keep your old resolver, because you'll be returning an object where masterCards is just a JSON array.