Persisting dynamic object in DynamoDB with .NET SDK

Question

I'm trying to persist the following class to DynamoDB using the .NET SDK:

public class MyClass
{
    public string Id { get; set; }

    public string Name { get; set; }

    public object Settings { get; set; }
}

The problem is with the Settings property. It can be any type of object, and I do not know in advance what might be assigned to it. When I try to persist it to DynamoDB, I get the following exception:

System.InvalidOperationException: 'Type System.Object is unsupported, it has no supported members'

Both the Document Model and Object Persistence Model methods result in the same exception.

Is there a way to persist these objects in DynamoDB? Other databases like MongoDB and Azure DocumentDB will do this without any issue, and they can be deserialized to either the proper type with a discriminator, or as a dynamic JSON object.

Try [converting Settings to a dictionary](http://stackoverflow.com/questions/4943817/mapping-object-to-dictionary-and-vice-versa) first and then try persisting the item to DynamoDB. — Alexander Patrikalakis, Mar 22 '17 at 22:36
The dictionary might have to be typed `IDictionary` though, right? — Jonn, Sep 04 '17 at 05:47
I can't believe there's no solution for this. DynamoDB is a document db and can't handle this? — JohnOpincar, Aug 30 '18 at 15:10

score 12 · Answer 1 · answered Aug 30 '18 at 16:07

You can use the general approach documented here: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBContext.ArbitraryDataMapping.html

Here's my implementation for any arbitrary object:

public class DataConverter : IPropertyConverter
{
    public object FromEntry(DynamoDBEntry entry)
    {
        var primitive = entry as Primitive;
        if (primitive == null || !(primitive.Value is String) || string.IsNullOrEmpty((string)primitive.Value))
            throw new ArgumentOutOfRangeException();
        object ret = JsonConvert.DeserializeObject(primitive.Value as string);
        return ret;
    }

    public DynamoDBEntry ToEntry(object value)
    {
        var jsonString = JsonConvert.SerializeObject(value);
        DynamoDBEntry ret = new Primitive(jsonString);
        return ret;
    }
}

Then annotate your property like this:

[DynamoDBProperty(typeof(DataConverter))]
public object data { get; set; }

I get the whole object serialized to string in the table instead of map. Not sure if I'm doing it incorrectly — LLL, Feb 13 '20 at 12:02
No, that's correct. DynamoDB doesn't support infinite (or even multi-level object depth). You can't even search a list of maps manually constructed. This solution is to store any arbitrary object in Dynamo. — JohnOpincar, Feb 19 '20 at 13:46

score 1 · Answer 2 · answered Dec 01 '21 at 03:42

Little improvement to the previous answer: make converter generic so that you can deserialize to the correct type, like this:

public class SerializeConverter<T> : IPropertyConverter
{
    public object FromEntry(DynamoDBEntry entry)
    {
        var primitive = entry as Primitive;
        if (primitive is not { Value: string value } || string.IsNullOrEmpty(value))
            throw new ArgumentException("Data has no value", nameof(entry));
        return JsonConvert.DeserializeObject<T>(value);
    }

    public DynamoDBEntry ToEntry(object value) => 
        new Primitive(JsonConvert.SerializeObject(value));
}

Usage:

[DynamoDBProperty(typeof(SerializeConverter<YourType>))]
public YourType data{ get; set; }

score 0 · Answer 3 · answered Mar 24 '22 at 22:17

I struggled to find a good solution for interacting with thoroughly unstructured data, then eventually realized that the DynamoDBContext really isn't designed for that.

For anyone else who gets to this point, my advice is to drop to a lower abstraction level and use the AmazonDynamoDBClient directly with Dictionary<string, AttributeValue> objects.

Persisting dynamic object in DynamoDB with .NET SDK

3 Answers3

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