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Amazon web services – Querying a single-table DynamoDb for child objects in .net

I have the following data structure in Dynamo and am able to execute queries according to my required access patterns, but I am attempting to use the higher level API to let the AWS framework take care of conversion/deserialization from the base dictionary types.

Dynamo Db Sample Data

When I query, I do get back all of the "rows" and fields that I expect, but I am unable to determine how to tell the framework that "this order has a collection of products"

DynamoDBContext context = new(client);
var order = context.QueryAsync<Order>("10594", new DynamoDBOperationConfig
{
    OverrideTableName = tableName,
});

This returns data in a way that makes sense conceptually, but isn’t what I’m trying to do.

enter image description here

public class Order
{
    [DynamoDBHashKey(AttributeName = "pk")]
    public string OrderId { get; set; }
    
    [DynamoDBRangeKey(AttributeName = "sk")]
    public string SortKey { get; set; }
    
    [DynamoDBProperty(AttributeName = "customerID")]
    public string CustomerId { get; set; }
    
    [DynamoDBProperty(AttributeName = "employeeID")]
    public string EmployeeId { get; set; }
    
    [DynamoDBProperty(AttributeName = "freight")]
    public decimal Freight { get; set; }
    
    [DynamoDBProperty("Products")]
    public List<Product> Products { get; set; }
    
}

Using the AmazonDynamoDbClient, I can get all the data back in a single call, but them I’m stuck mapping dictionaries to types. Is there a way to do this with the DynamoDbContext API, or when using approach should I be issuing multiple queries?

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2

Answers


  1. 0

    DynamoDB doesn’t support aggregation function such as group by and project like MongoDB. In order to achieve within DynamoDB, I would suggest storing one record for each order, so list of products can be stored in list directly in that record.

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  2. 0

    DynamoDB doesnt support that natively. I think that you can achive that via below sample.

    public class OrderItem
{
    [DynamoDBHashKey(AttributeName = "pk")]
    public string OrderId { get; set; }
    
    [DynamoDBRangeKey(AttributeName = "sk")]
    public string SortKey { get; set; }
    // Other common properties
}

public class Order : OrderItem
{
    // Properties specific to the main order
    public List<Product> Products { get; set; } = new List<Product>();
}

public class Product : OrderItem
{
    // Properties specific to product items        
}

// query all data 
DynamoDBContext context = new(client);
var order = context.QueryAsync<OrderItem>("10594", new DynamoDBOperationConfig
{
    OverrideTableName = tableName,
});
Order mainOrder = null;
List<Product> products = new List<Product>();
// loop through the list => determine your main order and the product based-on your sk
foreach (var item in items)
{
   if (item.SortKey == "ORDER")
   {
       mainOrder = item as Order;  // Cast if necessary and instantiate as Order
   }
   else if (item.SortKey.StartsWith("product#"))
   {
       products.Add(item as Product);  // Cast and add to products list
   }
}
// add product to your order object.
if (mainOrder != null)
{
   mainOrder.Products = products;
}
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