DynamoDBDocumentClient does not unmarshall attributes on ConditionalCheckFailedException

[brianle1301]

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• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow.
• I've searched for previous similar issues and didn't find any solution.

Describe the bug

When using the document client and setting ReturnValuesOnConditionCheckFailure: 'ALL_OLD', item attributes returned with ConditionalCheckFailedException are not unmarshalled

Regression Issue

• Select this option if this issue appears to be a regression.

SDK version number

@aws-sdk/lib-dynamodb@3.705.0

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v22.3.0

Reproduction Steps

Run the snippet below against a record that violates the condition expression, using your own values and attribute names if needed.

const client = DynamoDBDocumentClient.from(new DynamoDBClient());
try {
    const result = client.send(
        new UpdateCommand({
            TableName: 'MyTable',
            Key: { pk: 'pk', sk: 'sk' },
            UpdateExpression: 'SET my_attribute = :attr',
            ConditionExpression: 'attribute_exists(pk) AND owner_id = :ownerId',
            ExpressionAttributeValues: { ':attr': 'test', ':ownerId': 'ownerId' },
            ReturnValues: 'ALL_NEW',
            ReturnValuesOnConditionCheckFailure: 'ALL_OLD',
        }),
    );
} catch (error) {
    if (error instanceof ConditionalCheckFailedException) {
        console.log(error.Item); // error.Item is unmarshalled
    }
}

Observed Behavior

The record item returned along with the error is not unmarshalled

Expected Behavior

The record item returned along with the error should be unmarshalled

Possible Solution

Catch the error, unmarshall the item and rethrow

Additional Information/Context

No response

[RanVaknin]

Hi @brianle1301,

The SDK's behavior here is working as intended. The ReturnValuesOnConditionCheckFailure feature was specifically designed to return the raw item state at the time of failure - there's no automatic unmarshalling happening here, nor should there be.

Looking at the official announcement blog post (https://aws.amazon.com/blogs/database/handle-conditional-write-errors-in-high-concurrency-scenarios-with-amazon-dynamodb/), you can see that in the examples provided by the DDB team return the raw DynamoDB format. Check out their Python example:

Current Value: {'bacon': {'N': '4'}, 'lettuce': {'N': '0'}, ...}

The whole point of this feature is to avoid an extra GetItem call when a condition check fails - it's giving you the raw state so you can figure out why the condition failed. If you need the data unmarshalled, you'll have to handle that yourself by using the unmarshall functionality from import { unmarshall } from "@aws-sdk/util-dynamodb";

Hope this helps clear things up!

Cheers,
Ran~

[brianle1301]

@RanVaknin Hello,
Thanks for your reply.

I don't think there's a need for an extra GetItem call to get the item unmarshalled? I'd suppose the document client can catch the error, introspect it and if the Item key exists, unmarshall it.

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