How to update an item in Dynamodb of type String Set (SS)?

Question

I have created a attribute of type String Set. When I create the Item and assign an attribute of type SS everything works. But when I try to update this attribute, the data type changes to a list ("L").

I try this:

qw = new AWS.DynamoDB.DocumentClient();

var params = {
  TableName : "myTable",
  Key: {
    "id": somekey
  },
  UpdateExpression: "set ssvar= :arrp",
  ExpressionAttributeValues: {
     ":arrp": [ "test", "test2" ]
  }
};

qw.update (etc.)

This leads to a change in datatype in dynamodb and in stead of a string set I get a list:

"ssvar": {
            "L": [
                {
                    "S": "test"
                }, 
                {
                    "S": "test2"
                }
            ]
        }

I have tried all kinds of solutions like below but always my datatype gets changed.

  ExpressionAttributeValues: {
     ":arrp": 
        "SS": [ "test", "test2" ]
  }

How can I update an attribute of type string set?

Answer 1

As of September 2015, there is a createSet function in the DocumentClient that you can use for this.

UPDATE - added example

I've modified your example code to use this function:

qw = new AWS.DynamoDB.DocumentClient();

var params = {
  TableName : "myTable",
  Key: {
    "id": somekey
  },
  UpdateExpression: "set ssvar= :arrp",
  ExpressionAttributeValues: {
     ":arrp": qw.createSet([ "test", "test2" ])
  }
};

qw.update (etc.)

Answer 2

This is an artifact of using the DocumentClient - StringSet is not a JSON type.

The DocumentClient converts a StringSet to the Array native JavaScript type: https://github.com/aws/aws-sdk-js/blob/master/lib/dynamodb/converter.js#L61. Then the client serializes the native JavaScript Array as a DynamoDB List type: https://github.com/aws/aws-sdk-js/blob/master/lib/dynamodb/converter.js#L12.

If you want to use the StringSet type, you can use the low-level API: https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB.html