Dynamodb update attribute based on another

Question

I want to update an item in Dynamodb such that I can set the value of one attribute based on the value of an existing attribute in that item. For example: I have table with the following item {"id": 1, "valueone": 30}.

I want to update this item such that I can add another attribute valuetwo whose value is twice that of valueone: {"id": 1, "valueone": 30, "valuetwo": 60}

Something like this, but not sure how to represent valueone in the ExpressionAttributeValues:

table.update_item(Key={'id': 1}, UpdateExpression="set vtwo = :two * :r", ExpressionAttributeValues={':r': valueone, ':two': 2},ReturnValues="ALL_NEW"))

I can control whether valuetwo is a new attribute in the item or already exists with a dummy value (say zero).

Answer 1

Currently, DynamoDB doesn't support " * - Multiply (or) / - Division" arithmetic operators in UpdateExpression.

It supports only addition (+) and subtraction (-) operators.

Addition:-

UpdateExpression : "SET total_new_val = total_val + :value",

Subtraction:-

UpdateExpression : "SET total_new_val = total_val - :value",

Multiplication:-

UpdateExpression : "SET total_new_val = total_val * :value",    

Multiplication throws the below error:-

Unable to update item. Error JSON: {
  "message": "Invalid UpdateExpression: Syntax error; token: \"*\", near: \"tota
l_val * :value\"",
  "code": "ValidationException",
  "time": "2017-02-07T11:32:41.478Z",
  "requestId": "64f36024-7251-40af-98ee-e9cef854e94b",
  "statusCode": 400,
  "retryable": false,
  "retryDelay": 0
}