Nested Object Update issue - Appsync DynamoDB

I have been using appsync and dynamodb as datasource.

```
type Thing @model{
id: ID
title: String!
meta: String
thingData: ThingData
}

type ThingData {
firstName: String
lastName: String
}
```

Here Thing type is a DynamoDb table and ThingData is nested object field of Thing table. When I'm trying to update property firstname in ThingData, Object is replaced and lastname is removed. How to update individual property in nested objects?

The issue provides only the Thing and ThingData GraphQL schema; start by reproducing the firstName update against the AppSync DynamoDB datasource and compare the resulting Thing object. Done means determining how to update firstName while retaining the existing lastName, or clearly documenting the applicable limitation.

Written by the indexing model from the issue text.