dynamodbattribute.UnmarshalMap casts empty Lists to nil

[jcoyne]

Please fill out the sections below to help us address your issue.

Version of AWS SDK for Go?

v1.13.16

Version of Go (go version)?

1.9.3

What issue did you see?

When I have a record with an empty list in dynamodb, and I read it with the go sdk, and call dynamodbattribute.UnmarshalMap, then empty list values are cast to nil.

Steps to reproduce

Create an record with an empty list on the CLI:

aws dynamodb put-item --table-name resources \
--item '{"id": { "S": "555" }, "foo": { "L": [] }}'

Then fetch the record and unmarshal it:

        params := &dynamodb.GetItemInput{
		Key: map[string]*dynamodb.AttributeValue{
			"id": {
				S: aws.String(id),
			},
		},
		TableName:     aws.String(myTableName),
		ConsistentRead: aws.Bool(true),
	}
	resp, err := svc.GetItem(params)
        ...
        var json map[string]interface{}
	if err := dynamodbattribute.UnmarshalMap(resp.Item, &json); err != nil {
		panic(err)
	}
        log.Printf("JSON: %s", json)

The output is:

Item: map[id:555 foo:<nil>]

But I would expect:

Item: map[id:555 foo:[]]

If you have have an runnable example, please include it.

[jasdel]

Thanks for reporting this issue @jcoyne. I think this is a duplicate of the outstanding feature request #682. Generally empty lists are invalid DynamoDB attribute values. This is why the SDK uses NULL. Though #682 highlights this is not always the the case. +1'ing that feature request will help us prioritize it in the SDK's backlog.

[skmcgrail]

#2834 has been merged which implements a new (un)marshaler option for altering this behavior.