Updating nested object value in DynamoDB

In my recent project, I needed to update a single value in a nested object for a document on DynamoDB. In other projects, I would update the nested object property and then Save the whole document using SaveAsync from IDynamoDBContext. However, in this project, it needs to update the document from another microservice and I didn’t want to copy the model from that other microservice. This is because I only need to update one property and I don’t want this to have the knowledge of the whole model.

In this short post, I will show how I achieved this. Let’s say the model of our document looks like this.

    [DynamoDBTable("books")]
    public class Book
    {
        [DynamoDBHashKey]
        public Guid Id { get; set; }

        public string Name { get; set; }

        public Publisher Publisher { get; set; }
    }

    public class Publisher
    {
        public Guid Id { get; set; }

        public string Name { get; set; }
    }

Let’s say we need to update the name of the publisher for a given Document ID. I have a method to update it like this.

        private readonly IAmazonDynamoDB _ddbClient; // This will be injected via DI

        public async Task UpdatePublisherName(Guid bookId, string publisherName)
        {
            await _ddbClient.UpdateItemAsync(new UpdateItemRequest
            {
                TableName = "books",
                UpdateExpression = "SET #ps.#n = :name",
                Key = new Dictionary<string, AttributeValue>
                {
                    { "Id", new AttributeValue(bookId.ToString()) },
                },
                ExpressionAttributeNames = new Dictionary<string, string>
                {
                    { "#ps", "Publisher" },
                    { "#n", "Name" }
                },
                ExpressionAttributeValues = new Dictionary<string, AttributeValue>
                {
                    { ":name", new AttributeValue(publisherName) }
                }
            });
        }

The method does not need to reference the Book model. It only needs to know about the part it needs to update.